Heat the oven to 400° F.
Melt 2 Tbsp butter in skillet over medium heat. Add 2 Tbsp minced celery, 1 Tbsp each minced onion and red bell pepper. Cook 2 minutes, until tender; cool.
Remove stems from 2 lbs mushrooms; finely chop half the stems (about 1/4 cup). Reserve remaining stems for another use.
Combine celery mixture, chopped stems, 1cup oyster crackers, crushed, 6 oz cooked lobster meat, chopped, 1/4 pound cooked fresh crab meat, picked over and shredded, 1/4 cup shredded white Cheddar cheese, 1 large egg, 2 Tbsp water, Old Bay Seasoning, and 1/8 tsp each garlic powder, salt and pepper in bowl.
Grease two large jellyroll pans. Brush caps with 3 Tbsp olive oil. Sprinkle caps with salt. Stuff caps with lobster-crab mixture. Arrange caps in prepared pans. Divide and top with 2/3 cup shredded white Cheddar cheese. Bake 12 minutes, until lightly browned. Makes about 50 mushrooms.Makes 4 Source: