Peel the potatoes and slice them into thin rounds. Try to keep the slices even so they cook at the same time. Wash them lightly and set aside for later use.

Place the sliced potatoes in a saucepan with water. Boil for about 8–10 minutes until slightly tender. Do not overcook them. Drain and let them cool slightly.

Melt butter in a pan over medium heat. Add sliced onions and cook until soft and lightly golden. Stir often to prevent burning and bring out their natural sweetness.

Add flour to the onions and stir well. Slowly pour in milk while stirring continuously. Cook until the sauce thickens. This creates a creamy base for the pie.

Add grated cheddar cheese into the sauce. Stir until melted and smooth. Season with salt, pepper and nutmeg. Taste and adjust seasoning if needed.

Grease a baking dish lightly. Add a layer of potatoes, then pour some cheese sauce. Repeat layers until everything is used. Finish with sauce on top.

Sprinkle mozzarella cheese on top. This will create a golden and bubbly crust when baked. It adds extra flavor and texture to the pie.

Place the dish in a preheated oven at 180°C. Bake for about 30–35 minutes until the top is golden and bubbling. Let it rest for a few minutes before serving.