In an 8- to 10-inch nonstick frying pan over medium-high heat, boil shallots and 1/4 cup water until pan is dry. Stir shallots often until light brown, about 5 minutes.
Add wine, lemon peel, and lemon juice. Boil over high heat until reduced to 1/4 cup, about 5 minutes. Add milk, cereal, and 2/3 cup grapes (mixture will curdle) and stir until boiling.
In a blender, puree sauce. Season to taste with salt and pepper.
Arrange fish in a shallow 2 1/2- to 3- quart casserole. Pour sauce over fish.
Bake in a 450° oven until fish is opaque but still moist-looking in thickest part (cut to test), 8 to 10 minutes.
Rinse and dry frying pan. Place over medium-high heat. When hot, add remaining grapes. Shake pan until grapes are hot, about 2 minutes. Pour over fish.