Preheat the oven to 375˚F and grease a baking dish.
Heat the sausage in a large skillet over medium-high heat.
Cook the sausage until browned then stir in the mushrooms and onions.
Cook for 4 to 5 minutes then spread in the baking dish.
Sprinkle the dish with cheese then whisk together the remaining ingredients in a separate bowl.
Pour the mixture into the dish then bake for 35 minutes until bubbling.