Using a double broiler, heat mashed potatoes and cheese until the cheese is melted; add the garlic, salt, pepper, and chicken base. In a large skillet, melt margarine on medium heat; add potatoes to skillet and cook for ten minutes or until the potatoes are soft. Add the celery and onions to the skillet and cook an additional five minutes or until the celery and onions turn translucent. Remove from heat. Add sautéed ingredients to the broiler and simmer for twenty minutes.