Sweet Potato and Kale Gratin

I imagine this dish as going well with roasted chicken or turkey. The kale and the sweet potatoes complement each other well and the kale becomes pleasantly crispy under the broiler.
　
Ingredients
2 medium sweet potatoes
1/8 C brown sugar
7 T butter
¼ t nutmeg
Salt for boiling water
3 C chopped green kale
1 C diced onion
3 oz Emmenthal Swiss cheese, grated
3 oz Gruyere cheese, grated

Method
1. Bring a large stockpot with salted water to a boil. Wash and peel sweet potatoes. Cut into 1 inch cubes. Add to boiling water. Cook until sweet potatoes are tender, about 10-15 minutes.
2. While sweet potatoes are cooking, melt 5T of butter in a sauté pan. Add the onions and sauté over medium heat until softened. Add chopped kale and mix thoroughly. Cook until kale is wilted and softened, about 5 minutes.
3. Drain potatoes. Mash potatoes with brown sugar, nutmeg and 2T of butter.
4. Using a pie plate or shallow baking dish, spread the sweet potato mixture on the bottom of the dish. Spread the kale mixture evenly on top of the sweet potatoes. Sprinkle with cheeses.
5. Place dish under the broiler until cheese is melted and just slightly browned. Serve immediately.