Preparation
Preheat the broiler. Grease four individual 8 to 10 oz broiler-safe baking dishes. In a large skillet, melt 1 tablespoon of butter over moderate heat. Pour the butter into a small bowl; stir in bread crumbs. Set the buttered crumbs aside.
In the skillet, heat the remaining 2 tablespoons of butter over low heat. Add onion; saute until soft, 5 minutes. Increase the heat to moderate. Stir in scallops and mushrooms; saute for 5 minutes.
Add flour to the scallops and mix well. Gradually stir in wine and water. Bring the mixture to a boil, stirring constantly, until the sauce thickens, 2 to 3 minutes. Stir in half-and-half; heat the mixture until it is hot but not boiling. Remove the skillet from the heat.
Divide the scallop mixture among the baking dishes. Sprinkle each serving with an equal amount of cheese and top the cheese with the buttered crumbs. Broil 4 inches from the heat until lightly browned on top, 1 to 2 minutes. Serve immediately.