Caramelized Onion Garlic Butter Spaghetti Step-by-step instructions

Before you start

Before you start, set out all your ingredients and equipment. Bring a large pot of salted water to a boil for the spaghetti. Have your onions sliced, garlic shaved, cheese grated, and parsley chopped.

Measure the butter and olive oil, and keep the chili flakes and salt nearby. This is a quick-cooking sauce once the onions are caramelized, so having everything ready ensures a smooth process.

Instructions

Caramelize and Boil Pasta

1. 1Caramelize the Onions
   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.
   Look for Onions are soft, deeply golden, and smell sweet and nutty with no raw onion bite.
   Tip Don't rush the caramelization. If the onions brown too quickly, add a splash of water to the pan and lower the heat.
2. 2Cook the Spaghetti
   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.
   Look for The spaghetti is tender but still has a slight bite in the center.
   Tip Set a mug in the sink before draining so you remember to scoop out that pasta water.

The onions should be soft, deeply golden, and smell sweet and nutty. The spaghetti should be al dente, with a slight bite in the center.

You should have a cup of starchy pasta water reserved. The skillet should have a rich, buttery aroma with no raw onion smell.

Bloom Garlic and Build Sauce

3. 3Bloom the Garlic
   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.
   Look for The garlic smells toasty and nutty, not sharp, and has softened slightly.
   Tip Thinly shaving the garlic helps it melt into the sauce. Use a sharp knife or a mandoline.
4. 4Create the Emulsion
   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.
   Look for The liquid looks lightly thickened and coats the back of a spoon.
   Tip The sauce will look thin at first; don't worry, the starch thickens it as it simmers.

The garlic should be fragrant and just beginning to turn golden, not brown. The pasta water should be simmering and slightly reduced, with a glossy, silky appearance. The bottom of the skillet should be scraped clean of any browned bits.

Finish and Serve

5. 5Toss with Parmesan
   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.
   Look for The sauce is glossy and clings to the pasta, with no pool of liquid at the bottom.
   Tip Tossing off the heat prevents the cheese from clumping and keeps the sauce silky.
6. 6Finish and Serve
   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.
   Look for The pasta is glossy, the onions are jammy, and the kitchen smells buttery and garlicky.
   Tip Warm the bowls with hot water or in a low oven so the pasta stays hot longer.

The spaghetti should be coated in a glossy, creamy sauce with no liquid pooling. The parmesan should be melted and smooth, not clumpy. The parsley should be bright green and the pasta should be hot, twirlable, and inviting.

Common mistakes

The most common mistake is rushing the onions. Cooking them over too high heat can burn the sugars and make them bitter. If the pan gets too dry, add a splash of water and lower the heat.

Another pitfall is adding the cheese over high heat, which can cause it to clump. Always turn off the heat before adding parmesan and toss vigorously. Finally, don’t forget to reserve the pasta water; without it, the sauce won’t emulsify properly.

How to tell it’s done

The onions are done when they are deeply browned, soft, and jammy, with no raw bite. The sauce is ready when it has reduced slightly and coats the back of a spoon.

The finished dish should have a glossy, clingy sauce that coats every strand of pasta, with visible bits of caramelized onion and flecks of parsley. Taste and adjust seasoning before serving.