Vidalia® Sweet Onions Au Gratin

5 cups cooked G&R Vidalia® sweet onions (drained and chopped)
4½ cups grated cheddar cheese
½ cup self-rising flour
¼ teaspoon black pepper
½ teaspoon salt
6 tablespoons butter
Mix onions, 3½ cups cheese and remaining ingredients. Pour in two-quart casserole dish. Sprinkle 1 cup grated cheese on top. Bake in oven at 350 degrees for 30 minutes. Serves 10 to 12.