Preheat oven to 350 F degrees.
Trim stems off mushrooms and chop; retain for later.
Melt ½ c. butter in 13x9x2 pan or glass dish.
In skillet saute chopped mushroom stems in ¼ c. butter and add onion.
Add heavy cream and remove from heat.
Add breadcrumbs then crab meat.
Stuff each mushroom cap and place in cooking dish. Bake until mushrooms are tender.
Remove and add cheese and return to oven until cheese is melted.