Preheat oven to 350 degrees F.
Heat butter or oil in a small skillet over medium heat. Add onion and saute until translucent, about 5 minutes. Remove from heat and transfer to a large bowl.
Add all remaining ingredients to bowl and mix together until thoroughly combined.
Pour into a greased 3-quart casserole dish. Bake for 20 to 25 minutes or until bubbly. Let stand for a few minutes before serving.