Prepare au jus gravy mix according to package directions. Pour finished gravy into slow cooker, add roast beef. Set to low or warm and allow meat to heat through. (I allow at least two hours with crock pot on low)
Prepare french fries according to package directions, season to taste.
Saute drained mushroom pieces and onion in frying pan, set aside.
Layer beef, mushrooms and onion on hoagie bun sliced lengthwise. Top with two slices swiss cheese per sandwich.
Ladle au jus gravy into small cups for dipping sandwich and/or fries.
Plate sandwich with fries and au jus cup. Can also add a pickle spear (I prefer kosher dill spears).