In a bowl, whisk together eggs, milk, mustard, salt and nutmeg. Remove pie plate from refrigerator. Sprinkle cheese evenly on bottom of crust. Scatter onions on top. Pour egg mixture over cheese and onions. Bake quiche at 375 degrees F for 45 minutes, or until eggs are set and crust is browned. Cool 10 minutes on a wire rack. Serve with arugula, if desired.