Step 1

In a large pot of salted boiling water, cook pasta 1 minute less than package instructions for al dente. Reserve 1/2 cup pasta water; drain pasta. Set aside.

Step 2

While pasta cooks, heat oil over medium in a Dutch oven or 5-quart heavy pot with a lid. Add onion, garlic, and red-pepper flakes. Cook, stirring occasionally, until onion is soft, 5 to 7 minutes.

Step 3

Add wine, and bring to a boil; cook, stirring occasionally, until reduced by half, about 2 minutes.

Step 4

Add clams; cover, and simmer, shaking pot occasionally, until clams open wide, 3 to 5 minutes. (Discard any that havent opened after 5 minutes.)

Step 5

Add pasta to clam mixture in pot; continue to cook until pasta is al dente, about 2 minutes. Remove from heat, and stir in butter and parsley. Add some reserved pasta water to thin sauce if necessary; season with salt.