Heat the oven to 400° F.
Melt 2 tablespoons butter in skillet over medium heat. Add 2 tablespoons minced celery, 1 tablespoon each minced onion and red bell pepper. Cook 2 minutes, until tender; cool.
Remove stems from 2 pounds 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 ounces 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 tablespoons water, Old Bay Seasoning, and 1/8 teaspoon each garlic powder, salt and pepper in bowl.
Grease two large jellyroll pans. Brush caps with 3 tablespoons 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.