Place 1/2 the potatoes in a medium baking dish. Layer with 1/2 of the onion, garlic and cheese. Season with salt and pepper. Repeat layers, reserving cheese for last layer. Pour in milk to approximately 1 inch depth.
Bake 40 minutes in the preheated oven, stirring twice. Top with remaining cheese and continue baking 30 minutes, or until potatoes are tender.