In a 10 inch skillet, melt the 1/2 cup of butter. Add garlic and onion and saute until onions are translucent. Add scallops and cook 2 or 3 minutes. Season with salt, pepper, and white wine to taste. Add sufficient bread crumbs to prepare a moist stuffing.
Place each flounder fillet dark-side-up on a flat surface. Placing the scallop stuffing in the center of each fillet, divide evenly among the fillets. Fold both ends of each fillet over the stuffing, overlapping the ends. Pour the melted butter and the hot water into a 9x12 inch baking dish. Transfer the stuffed fillets to the baking dish and bake 20 minutes while you prepare a white sauce.
White Sauce: In a small saucepan, melt butter, then whisk in flour. Cook over low heat 2-3 minutes, whisking constantly. Then add milk, and salt, pepper, and white wine to taste. Increase heat to medium, whisking constantly until the sauce is thickened. Cook several minutes over low heat, stirring. When the flounder has baked 20 minutes, pour the white sauce over the stuffed fillets. Return the baking dish to the oven briefly and heat until the sauce begins to bubble.