In a 5-1/2- or 6-quart slow cooker, combine brown sugar, cola, lemon juice, and mustard. Add ham, turning to coat. Cover and cook on low-heat setting for 8 to 9 hours.
Transfer ham to a platter; cover to keep warm. In a small saucepan, stir together the water and cornstarch. Add cooking juices from cooker. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 2 minutes more. Stir in horseradish. Slice ham and serve with sauce. If desired, garnish with sage leaves. Makes 20 to 24 servings.