In a frying pan, brown and crumble sausage. Drain on paper towel and allow to cool. Saute mushrooms, season with pepper. Let cool. Spray 15x10 inch baking dish with cooking spray. In a large bowl, thoroughly combine milk and ricotta cheese. With electric beater, blend in flour, baking powder and salt. Add eggs and beat in until fully incorporated. Stir in cooled sausage, mushrooms, grated cheese, spinach and onions. Pour into prepared dish. Cover and refrigerate overnight. Preheat oven to 350 degrees. Uncover and bake for 60 minutes until golden brown around edges and center jiggles slightly. Cool 15 minutes until set.