In a large pot over medium heat, combine potatoes, carrot, celery, water and salt. Bring to a boil, then reduce heat, cover and simmer until potatoes are tender, 15 to 20 minutes. Once tender, stir in milk.
In a small bowl, combine melted margarine, flour, steak seasoning and pepper. Stir into soup, increase heat to medium, and cook, stirring, until thick and bubbly. Remove from heat and stir in cheese until melted. Let stand 5 to 10 minutes before serving.
