Preheat oven to 375°. In a Dutch oven over medium heat, cook sausage, crumbling meat, until no longer pink, 5-6 minutes; drain. Stir in tomatoes, spinach and 2 cups cheese., Pour sausage mixture into a greased 11x7-in. baking dish; top evenly with mashed potatoes. Bake 20 minutes; sprinkle with remaining cheese. Bake until cheese is melted and top begins to brown, about 20 minutes longer. Let stand 10 minutes before serving.