01 -
Pour the olive oil into a small saucepan. Add smashed and peeled garlic cloves, red pepper flakes, and fresh basil leaves. Set the pan over medium-low heat. When it starts to bubble, let simmer for 1 to 2 minutes, or until basil is wilted. Remove from heat and set aside to infuse.
02 -
Bring a large pot of salted water to a boil. Cook spaghetti until just al dente according to package directions. Before draining, reserve 1/2 cup of the pasta water.
03 -
Slice the onion in half through the root and peel away the skin. In a large heavy-bottomed pot over medium-low heat, melt butter with a splash of olive oil. Add the onion half, cut side down, and cook until golden brown on the cut side and butter becomes nutty, about 8 minutes.
04 -
Add canned tomatoes to the pot. If tomatoes are whole, break them up gently with a wooden spoon. Once the sauce reaches a boil, reduce heat. Season with salt. Cover tightly and simmer on low heat for 8 minutes.
05 -
Pour the infused basil oil into the tomato sauce and stir to combine thoroughly. Adjust seasoning with additional salt as needed. Add reserved pasta water to the sauce. Let simmer, uncovered, for another 5 minutes.
06 -
Remove the onion halves from the sauce. Using an immersion blender, blend the sauce directly in the pot until smooth and emulsified.
07 -
Add drained pasta to the sauce, tossing to coat the noodles thoroughly. Serve immediately in bowls, topped with freshly grated Parmesan cheese and additional fresh basil leaves if desired.