Preheat the oven to 350 degrees. Spray a 9x13-inch baking dish with cooking spray.

In a large mixing bowl, whisk the eggs with milk, ½ cup marinara sauce, Italian seasoning, salt, and pepper. Add the bread and 2/3 of the mozzarella cheese and gently stir. Let sit while preparing the rest of the recipe, stirring gently occasionally.

In a large skillet over medium-high heat, brown the sausage, breaking it up as it cooks. When cooked through and no longer pink, about 8 minutes, add the onion, mushrooms, and garlic. Cook, stirring occasionally, until the mushrooms and onions are tender, about 5 minutes. Add the sausage mixture to the bread mixture and stir gently to combine. Pour into the prepared baking dish and spread into an even layer.

Top the bread mixture with the rest of the marinara sauce and sprinkle on the rest of the mozzarella and the Parmesan cheese. Bake for 55 – 60 minutes or until gold brown on top and the filling is set. Let cool for 10 minutes before cutting.