Preheat oven to 325 degrees. In preheated oven, melt butter in a 9x13x2-inch baking dish;carefully tilt dish to coat bottom of dish with butter and set aside. In a large bowl, beat eggs, sour cream, milk, salt and basil until blended. Stir in cheese, mushrooms, green onions and pimientos; pour mixture into buttered baking dish. Bake, uncovered, for 45 minute, or until omelette is set in center but still moist. Allow omelette to set for about 5 minutes, and then cut into 12(3-inch) squares. Garnish with additional sliced green onions, if desired.