- Preheat oven to 375°. Butter a 10×15-inch baking dish.
- In a medium bowl, combine 1 egg, 1 heaping Tbsp. mayonnaise, 1 Tbsp. lemon juice, dry mustard, Worcestershire sauce and salt and mix until smooth.
- Sprinkle breadcrumbs over crab meat and toss gently to maintain any lumps. Add sauce mixture to crab and mix gently by hand.
- Stuff mushroom caps with crab mixture, mounding in the center and pressing gently to adhere (use about 2 spoonfuls per cap).
- Arrange stuffed mushrooms in a prepared dish. (Can be made 1 day ahead. Cover and refrigerate.)
- Bake at 375°F for 12 to 18 minutes, until crab meat begins to lightly brown and mushrooms begin to release juices.
Meanwhile, prepare Topping:
- Whisk remaining egg, 1 cup mayonnaise, 1 Tbsp. lemon juice, and half and half in a small bowl until thick and smooth.
- Remove mushrooms from oven and drizzle each with sauce (about 1 Tbsp. on each). Sprinkle with seafood seasoning.
- Return to oven and bake for an additional 6 to 8 minutes, until mushrooms are tender and sauce thickens slightly. Serve hot.
- Use a melon baller to scoop out the mushroom stems.
- Can be made 1 day ahead. Cover and refrigerate.