Preheat the oven to 350 degrees. Grease the 9x13 casserole dish with nonstick cooking spray.
In a medium skillet over medium high heat, melt the butter. Add the mushrooms and onion to the skillet. Cook for 5 to 6 minutes or until the vegetables are softened.
Mix in the sauce and stir until everything is evenly coated.
Transfer the mixture to the casserole dish and spread it evenly.
Bake uncovered for 30 minutes.
Top the casserole with the remaining fried onions and bake for 5 minutes more.
2 cups fried onions
Notes
Prepare the casserole ahead of time and refrigerate it to absorb flavor, making it more delicious. Cover tightly with plastic wrap or foil and place in the refrigerator until ready to bake.