Melt butter over medium heat in a large pan until golden brown, about 5-7 minutes. Add sliced yellow onion, stirring until translucent, about 3-4 minutes.

Stir in minced garlic and chili pepper flakes, cooking for another 1-2 minutes.

Pour in San Marzano tomatoes, crushing them with a spatula. Bring to simmer and cook for 15-20 minutes.

In a separate pot, boil salted water and cook spaghetti according to package instructions, about 8-10 minutes.

Once spaghetti is cooked, reserve pasta water, drain, and add to the sauce, tossing to combine.

Remove from heat and stir in chopped basil. Serve hot.