Spread English muffin halves with the 3 tablespoons butter. Line a greased 2-quart rectangular baking dish with half the muffins, buttered side up. In a medium bowl combine crabmeat and cheeses. Layer crab mixture over muffins in dish; sprinkle with capers. Top with remaining muffins, buttered side up.
In a large skillet cook onion in remaining 1 tablespoon butter over medium heat for 5 minutes or until tender. Stir in sherry and Worcestershire sauce. Bring just to boiling; remove from heat.
In a medium bowl combine eggs, milk, mustard, parsley, dash salt, and dash pepper. Stir in onion mixture. Carefully pour over the layers in baking dish. Press muffins lightly with back of a large spoon to moisten muffins on top. Cover; chill overnight.
Preheat oven to 350 degree F. Bake, uncovered, for 50 minutes or until muffins are browned and a knife inserted near the center comes out clean. Let stand 10 minutes before serving. Makes 8 to 10 servings.