12 ounces linguine
12 ounces cherry or grape tomatoes, halved or quartered if large
1 onion, thinly sliced (about 2 cups)
6 cloves garlic, thinly sliced
1/2- 3/4 teaspoon red-pepper flakes
2 sprigs basil, plus torn leaves for garnish
2-3 tablespoons extra-virgin olive oil, plus more for serving
kosher salt and freshly ground pepper
4 1/2 cups water