Melt the butter in a heavy-bottomed saucepan over very low heat. When the butter is completely melted, stir in the flour with a wooden spoon. Cook for about 1 minute over low heat, stirring constantly. Slowly add the milk. Using a whisk, stir briskly until you have a smooth sauce, about 2 minutes. Add the wine, lemon juice, salt, black pepper, and cayenne and whisk again. Switch to the wooden spoon and keep stirring the white sauce until it is completely smooth and thickened, about the consistency of mayonnaise. Remove from the heat.
Bring 2 cups water and 1/2 teaspoon salt to a boil in a medium saucepan and add the shrimp. When the water returns to a boil, cook the shrimp for 1 minute. Drain immediately. Roughly chop the shrimp and put in a large mixing bowl. Add the crabmeat and, with your hands, toss gently to mix.
Pour the sauce over the seafood. With a large spoon, gently combine, taking care not to break apart the crabmeat.
Spray an 8-inch square disposable aluminum foil pan with vegetable oil cooking spray (there will be a little left over for a tasty lunch for the cook) or use an 11-by-7-inch casserole dish. Pour the mixture into the pan. Place the grated cheese on top, completely covering the seafood mixture.
If freezing: Wrap the uncooked casserole securely with plastic wrap, the with aluminum foil. Place the pan into a plastic freezer bag and seal. Freeze.
When ready to bake: Completely thaw in refrigerator. When ready to bake, remove the foil and plastic wrap. Bake at 350° for about 25 to 30 minutes, until bubbly.