How to Make It
In a large enameled cast-iron casserole, heat the oil. Add the pancetta and cook over moderately low heat until crisp, 5 minutes. Add the onion, celery, fennel and garlic and cook, stirring, until softened, 7 minutes. Stir in the wine and simmer over moderately high heat until almost all of the liquid is evaporated, 3 minutes. Add the mussels and stock and bring to a simmer; cover and cook until the mussels open, 4 minutes. Stir in the crème fraîche.
Off the heat, stir in the lemon juice, parsley, marjoram and crushed red pepper. Season with salt and transfer to bowls. Serve with crusty bread.