Preheat your oven's broiler. Clean mushrooms, and remove stems, reserving them for later. Marinate the mushroom caps in Italian dressing for 15 minutes. Remove from marinade, and place into a baking dish.

Broil at least 6 inches from heat for about 10 minutes. Check occasionally to make sure they do not burn. Turn off the broiler, and preheat the oven to 400 degrees F (200 degrees C).

Melt butter in a skillet over medium-high heat. Chop reserved mushroom stems. Saute onion, garlic, mushroom stems, and celery until tender. Add the crabmeat, and heat until cooked through, about 5 minutes. Transfer the skillet mixture to a medium bowl, and mix in crackers, 1/3 cup of the cheese blend, and mix until well blended. Use some of the Italian dressing marinade to hold the mixture together if necessary. Pack the crab mixture into the cavities of the mushrooms. Season with salt and pepper to taste.