- With the rack in the middle position, preheat the oven to 180 °C (350 °F).
- Wash the potatoes and sweet potatoes, drain and slice them thinly with a mandolin. Do not rinse the sliced potatoes so that they retain maximum starch.
- Butter the sides and bottom of a 30 x 20-cm (12 x 8-inch) baking dish. Line the bottom of the dish with slices of sweet potatoes. Sprinkle with cheese and drizzle with cream. Season with salt and pepper. Cover with a layer of potatoes, drizzle with cream and sprinkle with chives. Continue with all the ingredients and season with salt and pepper between each layer. Cover with aluminum foil.
- Bake for about 1 hour or until the vegetables are tender.
- If necessary, brown under the broiler. Let stand for 10 minutes, cut into squares and serve.