Preheat oven to 350°F. Remove stems from mushrooms and set caps aside. Finely chop the stems.
In a skillet, melt 2 tablespoons butter and sauté chopped mushroom stems and onions until tender, about 5-7 minutes.
In a bowl, combine crabmeat and lemon juice, mixing gently.
Add the sautéed onion mixture, mayonnaise, breadcrumbs, parsley, Worcestershire sauce, salt, and pepper to the crab mixture. Mix well.
Stuff the crab mixture into mushroom caps, mounding slightly.
Sprinkle stuffed mushrooms with Parmesan cheese.
Place in a greased 15x10x1-inch baking pan.
Melt remaining 2 tablespoons butter and drizzle over mushrooms.
Bake uncovered for 20-25 minutes or until heated through and golden on top.
Serve warm immediately.