Preheat oven to 375 degrees. In medium skillet saute onion and butter for 2 minutes. Add mushrooms, lemon juice and salt and saute until liquid evaporates. Set aside.
In large bowl mix eggs, cream, nutmeg and pepper thoroughly. Stir in mushroom mixture and cheese. Pour into pastry shell and bake at 375 degrees for 30-35 minutes or until knife inserted 1 inch from edge comes out clean.