three potato au gratin

2 pounds potatoes – Yukon gold, purple, and sweet – sliced thin
2 pounds sweet onions, caramelized
1 cup freshly grated French or Swiss Gruyere cheese
1 cup crème fraiche or heavy cream
Salt

Preheat the oven to 350 F.

Layer half of the potatoes in the dish. Sprinkle with half of the cheese and then half of the crème fraiche. Sprinkle with salt. Add another layer, using the rest of the ingredients.

Bake, uncovered, until the gratin is crisp and golden on top, from 50 to 60 minutes. Serve immediately.