In a large heavy saucepan, melt butter over low heat. Slowly whisk in flour and continue whisking over low heat for 3 minutes. Whisk milk into flour mixture. Bring to a slow boil and continue stirring until thick (doesn’t take very long). Remove from heat and stir in dry mustard, Tabasco, wine, cheeses, onion and parsley. Fold in crabmeat and shrimp. Season to taste with salt and pepper.
Divide seafood mixture equally between each ramekin or shell; sprinkle about 1 teaspoon bread crumbs on top of each. Place ramekins on a cookie sheet and bake until bubbly and brown on top, approximately 20 to 25 minutes.
Variation: Can be cooked in a large greased casserole dish instead of individual ramekins. Takes about 5 to 10 minutes longer to cook.
Make ahead: Dish can be prepared but not baked earlier in the day, covered and stored in the refrigerator. Bring to room temperature before baking.