Line 23 cm quiche dish or pie pan with pastry. Trim excess pastry around edges. Prick bottom and sides with fork. Bake at 200ºC or 3 minutes; remove from oven and gently prick with a fork. Bake 5 minutes longer; cool. Melt butter in a medium skillet over low heat; add onion, mushroom, lemon juice and salt. Cook until liquid evaporates; set aside. Combine eggs, whipping cream, nutmeg, pepper and cheese in a medium mixing bowl. Stir in mushroom mixture. Pour into pastry shell. Bake at 190ºC or 30 minutes. Let stand 15 minutes before serving. Yields one 23 cm quiche.