Heat the oven to 375°F and butter a 9-by-13-inch baking dish with 1 teaspoon of the butter.
Spread the cooked rice evenly in the prepared dish.
Melt 2 tablespoons butter in a large skillet over medium-high heat and cook the mushrooms for 6 to 8 minutes, until browned and dry.
Add 1 tablespoon butter, stir in the flour and cook for 1 minute, then whisk in the milk and half-and-half gradually.
Stir in the sherry, paprika, salt and pepper and simmer for 4 minutes, until the sauce coats a spoon.
Fold the shrimp and crabmeat into the sauce, pour it over the rice and top with bread crumbs mixed with the remaining melted butter.
Bake for 25 to 30 minutes, until the center bubbles and the shrimp are opaque and reach 145°F.
Rest for 5 minutes before serving.