Stir cornstarch and water in a small cup until cornstarch dissolves; set aside. Bring milk and margarine to a boil in a saucepan, stirring constantly. Reduce the heat and whisk the cornstarch mixture into the milk mixture, bring to a simmer over medium heat. Cook and stir until the mixture is thick and smooth. Add the cheese to the milk mixture and stir until the cheese has melted and is well combined.
Prepare chili as directed on the can. Pour the cooked chili and the cheese sauce over the top of the cooked french fries.
