Heat the olive oil in a large skillet over medium heat. Sauté the onion until soft, about 5 minutes, then add the garlic and cook 30 seconds.

Crush the tomatoes by hand into the skillet, add salt and pepper flakes, and simmer uncovered for 20 minutes, stirring occasionally.

Remove from heat. Blend with an immersion blender until smooth, about 30 seconds.

Return to low heat, stir in the butter and most of the basil, and cook until the butter melts, about 1 minute. Taste and adjust salt.

Toss with freshly cooked pasta, divide into bowls, and top with reserved basil and Parmesan.