In a large skillet, heat the butter and oil. Add the onion, zucchini, and mushrooms. Cook over medium-high heat, stirring often, until the vegetables are softened and lightly browned, about 6 minutes. Add the scallops, shrimp and seafood seasoning and cook, stirring, until the seafood is just cooked through, 3 to 4 minutes. Remove from the heat.
Add the cracker crumbs and season to taste with salt and pepper. (The stuffing can be made up to 1 day ahead and refrigerated. Leftover stuffing can be frozen. Bring to room temperature or take the chill off in the microwave before stuffing the fish.)
Preheat the oven to 400 degrees F.
Brush a large glass baking dish generously with melted butter.
Season the fish fillets on both sides with salt and pepper. Arrange 4 of the fillets in the baking dish. Cover with stuffing, pressing it in to make an even layer, and top with the remaining fillets. If some of the seafood stuffing spills out into the dish, that's fine. Brush the fish with melted butter.
Bake in the preheated oven until the fish is opaque in its thickest part and the stuffing is heated through, 20 to 30 minutes, depending on the thickness of the fish. Garnish with lemon wedges for squeezing over the fish.
Recipe courtesy Brooke Dojny, The New England Clam Shack Cookbook, Storey Books, 2003