Butter 10x15-inch baking dish. Combine 1 egg, 1 heaping tablespoon mayonnaise, 1 tablespoon lemon juice, dry mustard, Worcestershire sauce, and 1/4 teaspoon salt in medium bowl; whisk to blend. In another medium bowl, combine crabmeat and breadcrumbs. Pour egg mixture over crab mixture and mix gently.

Stuff mushroom caps with crab mixture, mounding in center and pressing gently to adhere. Arrange stuffed mushrooms in prepared dish. (Can be made 1 day ahead. Cover and refrigerate.) Position rack in center ofoven and preheat to 375F. Bake until mushrooms begin to release juices, about 18 minutes.

Meanwhile, whisk remaining egg, 1 cup mayonnaise, 1 tablespoon lemon juice, and half and half in small bowl. Remove mushrooms from oven and drizzle each with sauce, then sprinkle with crab boil seasoning. Bake until mushrooms are tender and sauce thickens slightly, about 8 minutes. Serve hot.

* Crab boil is a seafood seasoning mixture sold at specialty foodsstores and in the seafood section of supermarkets.