Heat the oven to 400 degrees.
Melt a tablespoon of butter in a skillet over medium heat, and saute the celery, bell pepper and onion until tender. Cool in the fridge.
Remove the stems and gills from the mushrooms, and finely chop half of the stems into pieces.
In a mixing bowl, combine the sauteed vegetables, mushroom stems, cheddar cheese, Old Bay, garlic powder, salt, oyster cracker crumbs, pepper, water, crab meat and the egg.
Place the mushrooms into a buttered baking dish.
Add a spoonful of filling to the mushrooms.
Break the cheese slices into pieces and place on top of the mushrooms.
Bake for 12-15 minutes.
Let cool slightly before serving.