Pasta Pomodoro

Name: Pasta Pomodoro
Ingredients: 1 lb. spaghetti; 2 tbsp. extra-virgin olive oil; 1 small yellow onion, chopped; 3 cloves garlic, minced; 4 medium tomatoes, chopped (about 4 cups); Kosher salt; Freshly ground black pepper; Pinch crushed red pepper flakes; 2 tbsp. thinly sliced basil, plus more for serving; Freshly grated Parmesan, for serving