Slice the baguette into thin slices and let them dry out thoroughly for a few days, then grind them in a food processor. Melt the butter in saute pan and add the shallots and garlic. Cook them for 2 minutes, or until softened. Turn off the heat and add 2/3 cup fresh, not-too-fine bread crumbs, and the parsley and blend well. Season with salt and pepper.
Scrub and debeard the mussels. Put them in a large pot. Add enough wine to cover half the mussels. Cover the pot and turn up heat to high. When the wine starts to boil, shake the pan back and forth over the heat, just until mussels open. Remove cooked mussels to a large roasting pan and remove top shells. Loosen the mussel from remaining shell.
Top the mussels with bread crumb mixture. Place them under the broiler and broil them for 1 minute, or until mussels start to sizzle and brown.
Recipe courtesy of David Rosengarten