1. Preheat the oven to 500°. Melt the butter in a medium skillet. Add the garlic and shallot and cook over low heat until softened, 5 minutes. Transfer to a bowl and let cool. Stir in the mustard. Add the bread crumbs and thyme, season with salt and pepper and toss well.
2. In a medium saucepan, bring the wine to a boil with the bay leaves. Add the mussels, cover and cook over high heat until they open, 1 to 2 minutes. Transfer to a bowl; strain the cooking liquid into a cup and add a pinch of salt. Set the mussels on the half shell on a baking sheet; discard the empty mussel shells.
3. Set the clams on the half shell on a baking sheet. Put a shrimp in each of the empty clam shells and arrange on a baking sheet. Spoon ð teaspoon of the mussel liquid over each mussel, clam and shrimp to moisten them. Mound 2 teaspoons of the bread crumbs on the seafood in each shell.
4. Position 2 sheets of shellfish on a rack in the upper third of the oven and bake for 5 minutes, or until browned. Repeat with the third sheet. Serve.
Variation Spread the steamed mussels and raw clams and shrimp in a large gratin dish. Sprinkle with the seasoned bread crumbs and bake at 500° until browned. Toss with cooked linguine, olive oil and some of the mussel liquid.