Melt the butter in a large, heavy saucepan. Add the onion and cook over moderate heat, stirring, until just softened, about 2 minutes. Add the garlic and crushed red pepper and cook until fragrant, about 1 minute. Add the wine and bring to a boil. Add the mussels and about 1 cup of water. Cover and cook over high heat, stirring occasionally, until the mussels open, 8 to 10 minutes; discard any mussels that do not open. Spoon the mussels and broth into bowls and serve.