Chop mushroom stems. Butter a 13"x9" baking pan using one tablespoon butter. Place mushroom caps in prepared pan.
Melt remaining butter in a large skillet over medium heat. Add chopped stems, onion, seafood seasoning, salt and pepper; cook, stirring often, 5 minutes or until tender.
Combine crabmeat, egg and bread crumbs in a large bowl, stirring well. Add mushroom mixture and 2 tablespoons cheese, stirring well. Spoon, filling evenly, into mushroom caps; sprinkle with remaining cheese. Bake at 350° for 20 minutes or until golden.