Grill brats according to directions. When cool, cut into 1/4-inch bias slices. In skillet, sauté onion in butter until tender; set aside.
In small bowl, combine salad dressing and mustard. Slice French bread lengthwise and transfer to baking sheet. Spread salad dressing mixture on inside of bread.
Layer bottom of bread with six slices of cheese, brats, onions, sauerkraut and remaining cheese. Bake at 350 degrees for 10 minutes or until cheese is melted. Cut hoagie and serve hot.
Serving suggestion: To grill hoagie, assemble sandwich on a large sheet of heavy-duty foil. Wrap in foil. Grill, covered, over medium-low heat for 10 minutes or until cheese is melted.