Preheat oven to 350 degrees. Remove stems from mushrooms. Hollow out cap with spoon if needed. Place in muffin tins, one cap per cup.
In a medium bowl, combine softened cream cheese, crabmeat, green onions, garlic salt and lemon juice. Place a heaping spoonful of filling in each mushroom cap. Pour butter over caps and top with cheese.
Bake for 30 minutes or until cheese is melted and heated through.