PREHEAT oven to 400°F. Combine Ketchup, brown sugar and Mustard in medium saucepan. Stir in chicken and heat over medium heat, stirring occasionally, until hot.
ARRANGE French fries on baking sheet. Top with 1 cup cheese, chicken mixture, then remaining cheese.
BAKE 5 minutes or until cheese is melted. Sprinkle with chopped green onions if desired.