2 tablespoons unsalted butter
1 small leek, white and light green parts only, thinly sliced
1 small shallot, thinly sliced
4 medium cloves garlic, thinly sliced
2 bay leaves
Kosher salt and freshly ground black pepper
1 cup hard dry cider or white wine
2 pounds mussels (see note)
2 to 3 tablespoons homemade mayonnaise (see note), crème fraîche, or heavy cream (optional)
1 tablespoon juice and 1 teaspoon grated zest from 1 lemon
3 tablespoons minced fresh parsley leaves
Additional homemade mayonnaise for serving (optional, see note)
1 loaf rustic sourdough bread, thickly sliced, drizzled with olive oil, and broiled until heavily toasted