Preheat oven to 350°.
Bring water to a boil in a Dutch oven. Add kale and mustard greens, and cook 5 minutes or until tender, stirring occasionally. Drain; set aside.
Arrange 2 cups potato slices in a single layer in a 13 x 9-inch baking dish coated with cooking spray; top with 1 cup onion. Sprinkle with 1/4 teaspoon salt; top with half of kale mixture. Sprinkle kale mixture with 1/2 cup cheese. Repeat layers once, ending with the kale mixture. Top kale mixture with remaining 2 cups potatoes, and sprinkle with remaining 1/2 cup cheese. Pour broth evenly over potato mixture, and sprinkle with remaining 1/4 teaspoon salt. Cover with foil. Bake at 350° for 45 minutes. Uncover and bake 30 minutes or until lightly browned and potatoes are tender.