Melt butter in sauce pan. Remove and dice mushrooms stems. Dip caps in butter and place, rounded side down, on a rack on a cookie sheet. Drain clams and reserve liquid.
In melted butter, saute mushrooms stems and garlic. Add clam liquid and simmer until mushroom stems are tender. Remove from heat and stir in remaining ingredients. Spoon mixture into mushroom caps.
Broil about 6 " from heat for about 8 minutes, until mushrooms are tender and tops are lightly browned. Sprinkle a few drops of lemon juice on each and serve hot.