Mussels with Beer and Thai Stock

Name: Mussels with Beer and Thai Stock
Ingredients: 1 pound mussels, cleaned and de-bearded; 1 tablespoon butter; 1/2 cup onion, chopped; 1/3 cup lager-style beer; 1/2 cup Thai-flavored stock (store-bought or homemade; or just chicken stock); 1 tablespoon heavy cream; 3 tablespoons chopped fennel fronds; Kosher salt; Freshly ground black pepper; 4 pieces sourdough bread, toasted and buttered