Preheat oven to 400 degrees F. Saute celery, onion, and pepper in butter for 2 minutes, transfer to a plate and cool in the refrigerator. While vegetables cool, wash mushrooms and remove stems. Set caps to the side and finely chop half of the stems. Discard the other half of the stems or use elsewhere. Combine the sauteed vegetables, chopped mushroom stems, and all other ingredients (except cheese slices) and mix well. Place mushroom caps in individual buttered casseroles or baking dishes. Spoon 1 teaspoon of stuffing into each mushroom. Cover with sliced cheese. Bake in oven for 12-15 minutes until cheese is lightly browned.