In a large, deep skillet, heat the olive oil. Add the onion and garlic and cook over moderate heat, stirring occasionally, until softened, about 10 minutes. Add the thyme sprigs, wine and Pernod and bring to a boil. Stir in the mussels, cover and cook over moderate heat until they open, about 3 minutes. Transfer the mussels to a bowl, discarding any that do not open.