Preheat the oven to 400°F.
Wash the mushrooms and remove the stems, then finely chop the stems.
Melt butter in a skillet over medium-high heat.
Sauté the celery, onion, garlic, and bell peppers for 2 minutes, until tender.
Add the chopped mushroom stems and cook for another 2 minutes.
Transfer the mixture to a bowl and let it cool for 5 minutes.
Beat the egg and mix with crab meat, crushed crackers, ¼ cup cheddar cheese, Old Bay seasoning, salt, and pepper.
Fill the mushrooms with the crab mixture, top with the remaining cheese, and bake for 15 minutes. Serve warm.