Thinly slice the potatoes and onions.
Melt butter in a pan and sauté the onions.
Add heavy cream and milk, and bring to a simmer.
Layer the potato slices in an oven-safe dish.
Top the potatoes with the sautéed onions, sauce, and cheese.
Bake in a preheated oven at 200°C (392°F) for 25 minutes.