Bring a large saucepan of salted water to a boil. Place potatoes in the saucepan and cook until tender but firm, about 15 minutes. Drain and cool.
Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9x13 inch baking dish.
In a large bowl, mix the potatoes, onion powder, minced garlic, sour cream, cream of mushroom soup and Cheddar cheese. Pour into the baking dish. Sprinkle with mozzarella cheese.
Bake in the preheated oven 45 minutes, or until browned and bubbly.
