Step 1

Heat oil in a large skillet over medium-low. Cook onion, stirring occasionally, until soft, 10–12 minutes. Add garlic and cook, stirring, about 3 minutes. Add red pepper flakes and cook, stirring, 1 minute. Increase heat to medium, add puréed tomatoes, and season lightly with salt. Cook, stirring occasionally, until sauce thickens slightly and flavors have come together, 20–25 minutes. Remove from heat and stir in basil.

Step 2

Cook pasta in a large pot of boiling salted water, stirring occasionally, until very al dente (about2 minutes before it would be done). Drain, reserving ½ cup pasta cooking liquid.

Step 3

As soon as pasta is ready, pluck out and discard basil. Set skillet over high heat and stir in reserved pasta cooking liquid to loosen sauce; bring to a boil. Add pasta and cook, stirring, until sauce coats pasta and pasta is al dente, about 2 minutes. Remove pan from heat. Add butter and Parmesan and toss until butter and cheese are melted. Divide pasta among bowls.