Heat oven to 325º. Heat butter in 9x13 pan in oven until melted. Tilt dish to coat bottom. Beat eggs, sour cream or yogurt, milk, and salt until blended. Stir in onions. Pour into dish. Bake until eggs are set but moist, about 35 minutes (if desired, add grated cheese after 25 minutes.) Cut into serving pieces (12 to 16.)