Add water to a dutch oven and bring to a boil; add kale and mustard greens; cook 5 minutes or until tender, stirring occasionally; drain and set aside.
Arrange 2 cups potato slices in a single layer in a 13x9-inch baking dish that has been sprayed with nonstick cooking spray; top with 1 cup onion; sprinkle with 1/4 teaspoon salt; top with half kale mixture; sprinkle with 1/2 cup cheese.
Repeat layers once, ending with kale mixture.
Top kale mixture with remaining 2 cups potatoes and sprinkle with remaining 1/2 cup cheese.
Pour chicken broth evenly over potato mixture and sprinkle with remaining 1/4 teaspoon salt.
Cover with aluminum foil.
Bake in a 350°F oven for 45 minutes.
Uncover and bake 30 minutes or until lightly browned and potatoes are tender.