Brown ground beef with onion in a skillet until cooked through. Drain excess fat.

Melt butter in a saucepan, whisk in flour, then slowly add milk to form a smooth sauce. Season with salt and pepper.

Layer sliced potatoes and ground beef in a greased baking dish.

Pour sauce evenly over layers. Cover and bake at 375°F until potatoes are tender.

Uncover and bake until set and lightly golden. Rest before serving.