Slice the potatoes thinly.
Slice the onions thinly.
Cut the bacon into bite-sized pieces.
Melt the butter in a pan and sauté the onions and bacon.
Layer the potatoes in a heat-resistant dish.
Place the sautéed onions and bacon on top of the potatoes.
Pour in heavy cream and milk, and season with salt and pepper.
Generously top with mozzarella cheese.
Bake in a preheated oven at 180°C (356°F) for 25–30 minutes.