Spaghetti with caramelized onion and parmesan cheese

Spaghetti with caramelized onion and parmesan cheese

If you love the sweet and savory taste of caramelized onions, you’ll love this spaghetti smothered in caramelized onions and parmesan cheese recipe. I like to make the caramelized onions ahead of time and store them in the refrigerator. And the next day, when I can come home from work, I can whip up a tasty dinner in 15 minutes. This recipe is versatile and sophisticated, plus it looks impressive. The best part though is that it’s easy to make and tasty. Let’s start looking! 

About the ingredients:

Pasta: A long noodle is best for this dish. I like to either use spaghetti or fettuccine for this recipe because it twirls well with the caramelized onions. 

Onions: You can either use red or yellow onions for this dish because when cooked down, they have similar flavors. But if you’re looking for onions to not turn to mush when you cook them, opt for yellow onions. They are ideal for cooking because they hold up well to heat.

Parmesan: The creaminess of parmesan cheese elevates this dish, giving it an even richer flavor. I like to use high-quality parmesan for this recipe because there aren’t very many ingredients in the recipe, so the flavor of this cheese stands out. 

Soy sauce: Adding soy sauce to this dish adds more umami to the pasta sauce. If you don’t have soy sauce, you can substitute it with salt. 

Dijon mustard: With all these rich flavors, Dijon mustard adds a bit of tang to the dish. I recommend adding it to the sauce. It’s not mandatory, but it makes the pasta sauce sing. 

Thyme: Onions and thyme are a classic pairing. I love adding fresh thyme to this recipe, but you can also use dry if that's what you have. If you are adding dried thyme, cut the amount down by half when adding to this dish so the flavor doesn't overpower the other ingredients. 

Garlic: If you’re a garlic girl, then you’ll want to add two to four crushed garlic cloves to this recipe. Garlic adds spice and flavor to this dish.  

Spices: Freshly ground black pepper and crushed red pepper flakes boost the flavor of this pasta. The black pepper is essential for this recipe. If you don’t like spicy, feel free to omit the red pepper flakes.

Butter: A few tablespoons of butter to toast the garlic when you’re making the sauce helps add more richness to this dish. 

Olive oil: A quality cooking olive oil, not extra virgin, is necessary for breaking down and caramelizing the onions. Don’t skimp on this ingredient. Like the parmesan cheese, a good olive oil stands out in this recipe. 

Pasta water: Other than the pasta, pasta water is a key ingredient that makes the sauce velvety and smooth. It loosens the caramelized onions, melts the cheese, and brings the dish together. Make sure to reserve at least a cup of pasta water so that you can add as much as you need to make your sauce. 

Lemon juice: When serving this dish, adding a squeeze of fresh lemon juice enhances the flavors and cuts through the richness, giving the dish a sophisticated lightness. 

Why do onions caramelize?

The delicious caramelized onions you have on top of burgers or in a dip are made possible because of the interplay of chemistry and heat. Onions contain natural sugars called sucrose, and this sugar is a key part of the caramelization process. As you cook onions over low heat, their moisture starts to evaporate. This concentration of sugars sets the stage for caramelization. And by raising the heat and cooking the onions you trigger pyrolysis, which is a complex series of chemical reactions. The sugar molecules break down and rearrange, forming new compounds like caramels and melanoidins. These are responsible for the rich, deep brown color and the sweet, complex flavor characteristic of caramelized onions. The other reason onions caramelize is called the Maillard reaction, which is when food browns and deepens in flavor. You see the Maillard reaction in action when searing a steak or the crisping of pepperoni on pizza. The combination of caramelization and the Maillard reaction leads to a delicious result, caramelized onions, all you need is a little patience to make it happen.

Recipe: Spaghetti with caramelized onion and parmesan

  • Serves: 4 
  • Cooking onions: 1 hour
  • Pasta and sauce cooking time: 15 to 18 minutes

Pasta with caramelized onions and cheese served on a dish


  • ½ pound spaghetti
  • 6 medium onions (a combination of red and yellow onions) 
  • 1 tablespoon fresh thyme 
  • 1 teaspoon freshly grated black pepper 
  • ½ cup parmesan 
  • 1 tablespoon soy sauce 
  • Squeeze of fresh lemon juice 
  • 1 tablespoon  Dijon mustard 
  • ¼ teaspoon red pepper flakes 
  • 2 tablespoons unsalted butter 
  • 4 tablespoons olive oil
  • ½ cup water (to deglaze pan while cooking caramelized onions) 
  • Salt to taste 


Step 1

Slice 6 medium onions then set aside. Heat a Dutch oven or heavy bottom pan to medium heat, add in your cooking olive oil, and then add in your sliced onions. They will sizzle. Give them a toss in the onion with a wooden spoon and then leave them alone for 5 to 10 minutes to start breaking down. It will take about an hour to fully caramelize these onions.

Onions breaking down in the pan

You will need to stir every 5 to 10 minutes to see how the onions progressing. You will see fond form at the base of the pan when the onions are mostly caramelized. Fond is flavor. We want to keep those flavors. Therefore, add a little water to the onions to lift the fond.

Fond forming at the base of the pan

Start with a ¼ cup of water to deglaze the pan. The heat from the stove will help evaporate the water as you use your spoon to scrap up the fond and mix it into the onions. You may need to add more water to help lift the fond and finish cooking the onions. Make sure the water is fully evaporated and the onions are a rich dark color before you cut the heat. You can make these onions a few days ahead of time and store them in the fridge for up to 3 days. Or you can put them in the freezer so they can last longer, around 3 months.

 Caramelized onions

Step 2

Bring a large pot of water to a raging boil and add a tablespoon of salt. This salt helps flavor the pasta and raises the heat of the water. Add in your pasta, stir, and set the timer based on the pasta’s cooking instructions, but shave 1 minute off the cooking time because you’ll finish the pasta in the sauce. 

Pasta cooking in water

Step 3 

As the pasta cooks, you’re going to make your sauce. Add your butter and some olive oil to a seasoned pan. The heat should be medium-low. When the butter gets foamy, add in your sliced garlic and stir. You need to water the garlic so it doesn’t burn and get bitter. Once you smell the garlic, add in your spices, such as your crushed red pepper flakes and your freshly grated black pepper. 

Garlic, butter, and spices

Stir those spices into the butter and oil and then add in your caramelized onions and Dijon mustard. Mix everything until combined.  

Caramelized onions added to butter and garlic

Step 4

Once your pasta is cooked, al dente, use thongs to add the pasta to your pan with the caramelized onion sauce. Toss the pasta in the onion mixture and then ladle in your pasta water to help create the sauce.

Pasta added to onions

Mix everything, cut the heat, and then add in your parmesan cheese and fresh thyme, stir until combined and creamy.

Parmesan cheese scattered over pasta

This is when you give the pasta a taste to check for saltiness. If it is under-salted, add a few dashes of soy sauce and taste again. You can also omit the soy sauce and simply salt to taste at this point. 

Thyme added to the pasta

Step 5 

Serve up the pasta immediately and squeeze some lemon juice over the dish. I like to shave some parmesan on top and add a few sprigs of fresh thyme or red pepper flakes for color and contrast. Now, dig in! This pasta is great served right away, but it can last in the refrigerator for up to 3 days. Enjoy! 

Serve up your pasta with extra cheese and enjoy

Did you make this recipe? Let me know what you think by adding your comment below!

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