In a large bowl, combine oil, garlic, salt and pepper. Add potatoes and toss to coat; spread evenly in an ungreased 15-inch by 10-inch by 1-inch baking pan. Cover tightly with foil. Bake at 425° for 35-40 minutes or until tender. Cool at least 15 minutes. Meanwhile, in a large skillet, brown sausage and onion; drain. Combine spinach, ricotta, crumbs, cayenne, salt and pepper; mix well. Arrange a third of the potatoes evenly in a greased 13-inch by 9-inch baking dish. Layer with half of the spinach mixture, half of the sausage and half of the mozzarella. Repeat with a third of the potatoes and the remaining spinach, sausage and mozzarella. Pour broth over all. Top with remaining potatoes; sprinkle with Parmesan. Bake, uncovered, at 350° for 30-35 minutes. Let stand for 5 minutes before serving.