Mix all and divide and shape into 4 patties. Place in a baking dish. Bake at 400 degrees for 20 to 25 minutes, or until meat is done to your liking. While meat bakes, prepare sauce.
Sauce: Melt butter in a small saucepan; add flour. Over high heat, stir in milk; heat and stir constantly until thick. Stir in mustard, cheese and salt; stir until cheese is melted. Add tomato. Pour sauce over cooked meat patties. Top with a slice of cheese and a slice of tomato. Return to oven just long enough to heat tomato and melt the cheese, 3 to 5 minutes.