Spread English muffin halves with the 3 Tbsp. butter. Line a greased 2-qt. 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 the 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 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°F. Bake, uncovered for 50 minutes or until muffins are browned and a knife inserted near center comes out clean. Let stand 10 min. before serving.