Preheat oven to 400 and lightly butter a 9x13 baking dish. Spread half the cooked hash brown potatoes in the dish. Layer the spinach, onion, bacon and one cup of Monterey Jack cheese. Top with remaining potatoes, cheese and the sliced tomatoes.
Bake at 400 degrees for 15 to 20 minutes until slightly bubbly and cheese is melted. Mix the sour cream and horseradish in a bowl and set aside to serve on the potatoes.