Clean and stem mushrooms. Preheat oven to 350 degrees.
In a saucepan over low heat, melt half stick of butter. Add 1 tablespoon garlic. Spread half of this mixture in the bottom of a greased sheet pan or casserole dish. Place mushrooms, caps down, in the dish. Spoon a small amount of the mixture into each mushroom cap, leaving room in each for additional stuffing. Bake at 350 degrees for 10-15 minutes, depending on their size.
In a separate mixing bowl, combine cream cheese, 3/4 package crab, 1/2 diced onion, 1 tablespoon garlic and salt. Stuff mushrooms with mixture.
Bake at 350 degrees for 20 minutes. Remove from oven and top with mozzarella cheese. Put back in the oven until cheese melts.