Caramelized Onion Garlic Butter Spaghetti Tips, FAQ & printable recipe

Tips for success

Use a large skillet so the onions have room to cook without steaming. If the skillet is too crowded, the onions release moisture and end up boiling instead of caramelizing.

A wide, heavy-bottomed pan distributes heat evenly and gives the onions more surface contact. Stir occasionally but not constantly; letting them sit helps develop color.

When you add the pasta water, it will look thin, but resist the urge to add more pasta or cheese to thicken it. The starch in the water and the fat from the butter will emulsify as you toss, creating a silky sauce.

If the sauce seems too thin after tossing, simmer it for a minute longer, but don’t over-reduce or it will become sticky.

Variations

• Add Crispy Bacon: Cook 4 slices of chopped bacon until crisp, then remove from the pan and leave 2 tablespoons of drippings. Caramelize the onions in the drippings instead of butter and oil. Stir the bacon back in with the pasta and cheese for a smoky, salty twist.
• Make it Vegetarian with Mushrooms: Slice 8 oz cremini mushrooms and sauté them with the onions after they have caramelized. Cook until the mushrooms release their liquid and turn golden. The earthy flavor pairs beautifully with the garlic butter sauce.
• Go Gluten-Free: Use gluten-free spaghetti or other long pasta. Make sure to cook it in salted water and reserve some of the cooking water; gluten-free pasta may need a little more starch, so you can add a teaspoon of cornstarch to the water if the sauce seems thin.
• Spicy Arrabbiata Style: Increase the red chili flakes to 1 teaspoon or more, and add a can of diced tomatoes with the pasta water. Simmer for 5 minutes to create a spicy, tomatoey sauce that still has the caramelized onion base.

Storage and make-ahead

Fridge

Store leftovers in an airtight container in the refrigerator for up to 3 days. Reheat gently with a splash of water.

Freezer

Freeze in an airtight container for up to 2 months. Thaw overnight in the refrigerator before reheating.

Reheat

Reheat in a skillet over medium-low heat with a splash of water or broth, tossing until hot and creamy.

Leftover spaghetti can be stored in an airtight container in the refrigerator for up to 3 days. The sauce will thicken as it cools.

To reheat, place the pasta in a skillet over medium-low heat with a splash of water or milk, and toss until hot and creamy. Avoid microwaving for too long, as it can make the pasta mushy.

You can caramelize the onions up to 3 days ahead. Let them cool completely, then transfer to an airtight container and refrigerate. When ready to cook, reheat the onions in a skillet with a little butter or oil, then proceed with the recipe. The garlic and chili can be added when you reheat.

Serving ideas

Serve the spaghetti in warm bowls, twirled into neat nests. Top with extra grated parmesan and a sprinkle of fresh parsley. A drizzle of good olive oil just before serving adds a fruity note that contrasts with the richness.

Pair with a crisp green salad dressed with lemon vinaigrette and a glass of dry white wine like Pinot Grigio or Sauvignon Blanc.

For a more substantial meal, add a simple side of sautéed spinach or roasted broccoli. The pasta is rich enough to stand alone, but the bitter greens balance the sweetness of the onions. Crusty bread is also welcome to mop up any extra sauce from the bowl.

Frequently asked questions

Recipe card

Caramelized Onion Garlic Butter Spaghetti

Prep 10 minCook 25 minTotal 35 minServes 4

Ingredients

• 12 oz spaghetti
• 2 yellow onions, thinly sliced
• 3 tbsp unsalted butter, divided
• 1 tbsp olive oil
• 1/2 tsp kosher salt, plus more for pasta water
• 6 cloves garlic, thinly shaved
• 1/4 tsp red chili flakes
• 1 cup starchy pasta water, reserved
• 1 cup freshly grated parmesan cheese
• 1/4 cup fresh parsley, chopped

Instructions

1. Heat a large skillet over medium heat. Add the olive oil and 2 tablespoons of the butter. Once the butter is melted and foaming, add the sliced onions and the 1/2 teaspoon of salt. Cook, stirring occasionally, for 15 to 20 minutes, until the onions are deeply browned, sweet, and jammy. Reduce the heat if they start to scorch; a little patience here builds the base of the whole dish.
2. Bring a large pot of generously salted water to a boil. Add the spaghetti and cook according to package directions until al dente. Before draining, reserve 1 cup of the starchy pasta water. Drain the spaghetti, but do not rinse it; the residual starch helps the sauce cling.
3. When the onions are caramelized, push them to one side of the skillet. Add the remaining 1 tablespoon of butter to the empty side. Once melted, add the shaved garlic and red chili flakes. Cook for 1 to 2 minutes, stirring constantly, until the garlic is fragrant and just beginning to turn golden. Stir everything together.
4. Pour the reserved pasta water into the skillet. Stir well, scraping up any browned bits from the bottom. Let the liquid simmer for 2 to 3 minutes until it has reduced slightly and become glossy and slightly thickened. This starchy water emulsifies with the butter to create a silky sauce that clings to the pasta.
5. Add the drained spaghetti to the skillet. Turn off the heat. Sprinkle in the grated parmesan and use tongs to toss everything together vigorously, lifting and turning the pasta to coat every strand. Add a splash more pasta water if the sauce seems too thick. The cheese will melt into the emulsion and create a creamy, glossy coating.
6. Stir in the chopped parsley, reserving a little for garnish. Taste and adjust seasoning with salt and extra chili flakes if desired. Divide the spaghetti among warm bowls, twirling each portion into a nest. Top with extra parmesan and a sprinkle of parsley. Serve immediately while hot and glossy.

Contains: gluten, milk

Estimated nutrition per serving

Calories

510

Protein

18 g

Carbohydrates

72 g

Fat

17 g

Fiber

5 g

Sugar

8 g

Automatic estimate from the ingredient list, not laboratory-verified.