Heat oven to 325°F. Spray 12x8-inch (2-quart) glass baking dish with cooking spray. In 8-inch skillet, melt butter over medium heat. Add bell pepper and onion; cook 2 to 3 minutes, stirring occasionally, until tender.
In large bowl, beat eggs, sour cream, salt and pepper with wire whisk until well blended. Stir in bell pepper mixture and cheese. Pour into baking dish.
Bake uncovered 20 to 25 minutes or until eggs are set. Cut omelet into squares to serve.