Mix sage, salt and pepper in a cup; set aside. Layer 1/3 of potatoes and half of onion in a lightly greased 13"x9" baking pan. Sprinkle with one teaspoon of sage mixture and 1/3 of cheese. Repeat layers with remaining ingredients, ending with cheese. Whisk cream and broth together until well blended; pour evenly over top. Bake, covered, at 400 degrees for one hour, until tender and golden. Let stand 5 minutes before serving.