1. Peel the potatoes, wash them, dry them, and cut them into thin slices. Peel and slice the onion, mix it with the potatoes, season with salt and pepper.
2. Place one of the puff pastry sheets in the bottom of a pie dish, fill it with the potatoes.
3. Add half of the heavy cream. Cover everything with the second puff pastry sheet, sealing it with the first one, then brush the top pastry with an egg yolk.
4. Make a central chimney.
5. Bake in the oven (375°F) for about 1 hour.
6. Serve with the remaining cream.