With the rack in the middle position, preheat the oven to 180°C (350°F).
Butter a 20-cm (8-inch) square Pyrex baking dish.
Line the bottom of the dish with 1/3 of the potatoes. Season with salt and pepper. Cover with half the leeks. Cover with another 1/3 of the potatoes, then the remaining leeks and finally the last 1/3 of the potatoes, seasoning between each layer. Add the cream and milk. Top with cheese.
Bake until the vegetables are tender, about 75 minutes.
Cover and let stand for 15 minutes before serving, to make it easier to cut into portions.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.