12 ounces linguine
12 ounces cherry or grape tomatoes, halved or quartered if large
1 onion, thinly sliced (about 2 cups)
4 cloves garlic, thinly sliced
1/2 teaspoon red-pepper flakes
2 sprigs basil, plus torn leaves for garnish
2 tablespoons extra-virgin olive oil, plus more for serving
Coarse salt and freshly ground pepper
4 1/2 cups water
Freshly grated Parmesan cheese, for serving
Combine pasta, tomatoes, onion, garlic, red-pepper flakes, basil, oil, 2 teaspoons salt, 1/4 teaspoon pepper, and water in a large straight-sided skillet. Bring to a boil over high heat. Boil mixture, stirring and turning pasta frequently with tongs, until pasta is al dente and water has nearly evaporated, about 9 minutes.
Season to taste with salt and pepper, divide among 4 bowls, and garnish with basil. Serve with oil and Parmesan.