Winter Potato Gratin with Kale and Apples

Name: Winter Potato Gratin with Kale and Apples
Ingredients: 2 large yukon gold potatoes thinly sliced; 1 gala apple cored, thinly sliced; 1 bunch kale roughly 2 cups, de-stemmed; a pinch of salt and freshly ground black pepper; 1 tsp crushed dry sage; 1 tsp chili powder; 1 cup parmesan cheese finely grated; 2 cups low-fat 1% milk; 2 tablespoons butter