Preheat oven to 350°. In a large skillet, cook and crumble sausage over medium heat until no longer pink, 6-8 minutes; drain. Mix hash browns and cheddar cheese. Layer an 11x7-in. baking dish with half of each of the following: hash brown mixture, sausage, spinach and tomatoes. Repeat layers; sprinkle with mozzarella cheese. Whisk together eggs, milk, salt and pepper; pour evenly over top.
Bake, covered, 35 minutes. Sprinkle with Parmesan cheese. Bake, uncovered, until bubbly and cheese is melted, 10-15 minutes. Let stand 5-10 minutes before serving.
Make-Ahead: Refrigerate unbaked casserole, covered, several hours or overnight. To use, preheat oven to 350°. Remove casserole from refrigerator while oven heats. Bake as directed.