Heat oven to 350°F.
Spray 1 1/2 quart casserole dish with cooking spray.
In a 10 inch non stick skillet, melt butter over medium to 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.
*You can substitute 2 cups frozen whole kernel corn and 1/2 cup chopped red bell pepper for the Mexicorn®. Cook the bell pepper with the onion and then continue as directed.*