In a large saucepan, brown beef and bacon; drain if necessary. Stir in potatoes, onion, celery, green pepper, salt and water. Cover and cook until vegetables are tender, about 15-20 minutes.
Whisk together flour with 1/2 cup of the milk; add to saucepan along with remaining milk. Cook and stir until thickened and bubbly. Add cheese; heat and stir until cheese is melted. Garnish with additional cheese and sliced scallions, if desired.