Wash the sweet potatoes thoroughly, peel them, and slice them thinly.
Blanch the sliced sweet potatoes briefly in boiling water.
Finely chop the onion and bacon.
Melt butter in a pan and sauté the onion and bacon.
Place the blanched sweet potatoes and sautéed ingredients into a gratin dish.
Mix heavy cream, milk, salt, and pepper together, then pour over the sweet potatoes.
Generously top with mozzarella cheese.
Bake in a 180°C oven for 20 minutes.