Start by gently heating olive oil in a small saucepan. Add minced garlic, a sprinkle of red pepper flakes, and fresh basil. Warm it over low heat until fragrant, about 3-5 minutes.

In a separate pan, melt a tablespoon or two of unsalted butter over medium heat. Add a halved onion and let it brown slowly until golden and caramelized, approximately 8-10 minutes.

Toss the San Marzano tomatoes into the pan with the browned onion, mixing to combine the flavors. Allow this to cook for about 5 minutes.

Stir occasionally as you let the sauce simmer for another 5 minutes. Aim for a silky texture, perfect for clinging to the spaghetti.

Mix the infused basil oil into the sauce, adjusting the seasoning with salt and pepper to taste.

Toss the cooked spaghetti directly in the sauce. Ensure every strand is glossy and well-coated, then plate up and enjoy your delicious meal!