1. Preheat oven to 375°F (190°C). Grease a 9×13-inch baking dish.
2. In a skillet, brown ground beef with onion and garlic. Season with salt and pepper, then set aside.
3. In a saucepan, melt butter over medium heat. Stir in flour and cook for 1 minute. Gradually whisk in milk until smooth and thickened. Stir in half the cheese.
4. Layer half the potatoes in the baking dish, followed by half the ground beef and half the cheese sauce. Repeat layers.
5. Sprinkle remaining cheese on top. Cover with foil and bake for 40 minutes. Remove foil and bake another 15-20 minutes until golden and bubbly.
6. Let cool slightly before serving.