On medium heat put a drop of olive oil in your pan just so the mushrooms don't stick. Don't put a lot of oil as you know mushrooms let out a lot of water. Put mushrooms in pan for about 5 minutes just so they cook a little bit. Remove mushrooms and set aside.
In the same pan put your onions,celery,garlic,salt and pepper and cook until veggies are tender. Then put in your crab meat,stuffing mix and chicken broth. Mix it all together and if you feel its not wet enough just add more broth. Add in cheese and mix one more time. Put your mushrooms on a baking dish,put stuffing on top of the mushrooms I always put a heaping serving spoon amount on each and I top them with extra Mozzarella. Put them in the oven at 350 just so all the cheese melts. I would say for about 25-30min.