Heat oven to 350°F. Spray 1 1/2-quart casserole or soufflé dish with cooking spray. In 10-inch nonstick skillet, melt butter over medium-high heat. Add onion; cook 2 to 4 minutes, stirring occasionally, until crisp-tender.
Stir in remaining ingredients until well blended. Pour into casserole.
Bake uncovered 45 to 55 minutes or until set and knife inserted 1 inch from edge comes out clean.