Preheat oven to 400. Clean mushrooms, removing stems.
In blender or food processor, mix the butter, garlic, shallots, and parsley. Blend.
Add lemon juice, salt and pepper and blend briefly again. Put the mushrooms cap side up in a shallow baking dish. Fill each mushroom with a dollop of garlic butter mixture, about 1/2 to 1 tsp. each.
Bake for 10 to 12 minutes.