Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius). Coat a 9-inch round baking pan with nonstick cooking spray.

Heat oil in a skillet and sauté onions till soft, about eight minutes.

Add mushrooms and sauté for an additional two minutes.

In a large bowl, combine mushroom-onion mixture with remaining ingredients. Stir well to combine.

Pour mixture into prepared pan. Bake for 45–60 minutes, until top is lightly browned.