Mussels with Pancetta and Crème Fraîche

Name: Mussels with Pancetta and Crème Fraîche
Ingredients: 2 tablespoons extra-virgin olive oil; 4 ounces pancetta or bacon, finely chopped; 1/2 red onion, finely chopped; 1 celery rib, finely chopped; 1/2 fennel bulb, chopped; 4 garlic cloves, thinly sliced; 3/4 cup dry white wine; 48 mussels, scrubbed; 1 1/4 cups fish stock, or 3/4 cup clam juice plus 1/2 cup of water; 1/2 cup crème fraîche; 2 tablespoons fresh lemon juice; 1/2 cup packed parsley leaves; 2 tablespoons marjoram leaves; Pinch of crushed red pepper; Kosher salt; Crusty bread, for serving