Preheat a panini style grill. Lightly grease with nonstick spray.
Heat a saucepan on medium heat. Add the in the broth, kitchen bouquet, and steak seasoning. Bring to a low simmer to allow the flavors to come together. Taste and adjust the seasoning as needed. Whisk the cornstarch mixture in to thicken the broth just to give it body (just thickened to coat the meat, not thick like a gravy). Add the roast beef into the saucepan and low simmer for 5 minutes.
Using tongs or a slotted spoon place the beef on the bottom half of the bread and place the cheese on top. Place on the panini grill and cook until golden brown and the cheese is melted.(Alternatively, cook in a nonstick pan on medium heat using a spatula to press down on both sides to golden brown and the cheese has melted fully).
Cut in half and serve with chips and pickle. Serve the au jus on the side.