Combine the wine, water, salt, pepper, and instant onion in saucepan, simmer for 5 minutes. Add scallops and mushrooms, cover, simmer for 5 minutes. Remove scallops and mushrooms, set aside, while preparing sauce. Cook stock in pan until reduced to 1/2 cup, about 15 minutes. Melt the butter in another saucepan, stir in flour. Stir in fish stock and milk, cook and stir until mixture thickens and bubbles.  Add cheese, stirring until melted. Season with salt and pepper, if needed. Remove from heat, add scallops and mushrooms. Spoon into two individual baking dishes. Bake in moderate oven at 375ºF. for 15 to 20 minutes. Trim with parsley. Serves 2