Grill brats according to package directions. When cool enough to handle, cut into 1/4 in bias slices. In a skillet, saute onion in butter until tender, set aside. In a small bowl, combine salad dressing and mustard.
Slice french bread lengthwise and transfer to a baking sheet. Spread salad dressing mixture on the inside of bread. Layer bottom of bread with six slices of cheese, brats, onions, sauerkraut and the remaining cheese. Bake at 350 degrees for 10 minutes or until cheese is melted. Cut hoagie and serve hot.