- Preheat oven to 180°C (350°F). In a medium saucepan, cook butter and leeks over medium heat until softened, about 3 min. Stir in cream, broth and thyme; increase heat to bring just to a boil. Reduce heat to medium and simmer until liquid reduces to about 1 1/3 cups (325 mL), about 5 min. Stir in pepper, nutmeg and salt. Set aside.
- Using a sharp knife or mandolin, thinly slice potatoes into rounds (about 1/8-in./3-mm thick). In a 2-qt (2-L) baking dish, starting with yellow-fleshed potatoes as the base, make a layer of overlapped slices. Alternate with sweet potatoes in next layer (Note: Change the direction of the overlaps with each layer to fit slices more snugly). Continue layering the two kinds of potatoes until all slices are used up.
- Pour the reserved sauce overtop. Cover dish with foil. Bake 40 min., or until potato layers are tender when pierced. Uncover, sprinkle with cheese. Bake until cheese is melted and bubbly, about 20 min.
Recipe is suitable for reheating. Cover with foil, bake at 180°C (350°F) or higher until heated through, about 35 min.