Layered Two-Potato Casserole

4 TB butter (1/2 stick), melted
2 small sweet potatoes, thinly sliced
4 red potatoes, thinly sliced
1 tsp thyme
1/2 tsp salt
½ tsp black pepper
1 leek: white part finely chopped
4 TB grated Parmesan

1. Preheat oven to 425
2. Toss sweet potatoes with 2 TB butter, ½ tsp thyme, 1/4 tsp salt, & ¼ tsp pepper
3. Toss red potatoes with remaining butter, thyme, salt & pepper
4. Spray bottom of baking dish with cooking spray. Place one layer of sweet potatoes in overlapping circles on bottom of dish; sprinkle with some leeks; top with layer of red potato slices; sprinkle with 2 TB parmesan.  Repeat layers.
5. Cover & bake 40 minutes or until tender