Here’s what we did. Saute two strips of bacon, remove them from the pan and drain the excess fat. Saute an onion, a full bulb (or two, even) of garlic, and a handful of fennel, all finely chopped, in the bacon fat. When they’re browned and lovely, add back the chopped bacon (you chopped it while the other things were cooking) and add a few tablespoons of dry sherry and let it cook down. Next come your other liquids. We used about a cup each of vermouth, clam juice, and chicken stock, but the proportions and the specifics are flexible.
We adjusted the flavor with a little marsala and about a tablespoon of butter, brought the whole thing to a boil, tossed in about 3 pounds of mussels, and covered the lot. They opened in just a few minutes, and we ate them with a crusty bread.