In a large saucepan, mix together the Au Jus sauce, using all ingredients to taste. (I keep onion soup mix in an old spice jar to use as a seasoning) I start with 8 oz. of water, 2 teaspoons pepper, 1 tsp Worcestershire, 8 oz. Au Jus, 2 tsp soy sauce, 2 tbsp of onion soup mix and a dash of cayenne. Simmer.
In a skillet, melt butter then combine chopped green peppers, onions and mushrooms with black pepper. Add soy sauce and Worcestershire. Sauté on medium heat until veggies are tender crisp.
Slice the leftover prime rib so that it is at a 1/2 inch (or less) thickness, then set in the simmering Au Jus.
Cut French bread diagonally, toast pieces in toaster.
Spread french onion dip on both pieces (top and bottom).
Layer on prime rib from Au Jus.
Put sauteed mushroom, onion, green peppers on top of meat.
Cover with pepper jack cheese.
Broil sandwich open-faced for 2-3 minutes or until cheese is slightly golden.