Melt butter over medium heat. Add mushrooms and cook, stirring occasionally until tender and pan is dry, about 10-15 minutes. Transfer to plate and set aside.
Bring 3 quarts of water and bring to a rolling boil.
Combine 1 quart of cool tap water and 1 package of Gravy Mix in a separate container. Mix with a wire whisk until lump free.
Pour Gravy Mix into boiling water. Stir until gravy returns to a boil. Simmer for one minute while stirring constantly. Remove from heat and stir in cooked mushrooms. Maintain internal gravy temperature at 165°F during service.
Meanwhile, preheat grill to high heat. Season steaks with salt and pepper. Grill steak for 4-5 minutes per side, until internal temperature reaches 145°F for medium, or until desired internal temperature is reached. Allow to rest 5 minutes before slicing.
Top bottom half of each baguette with 2 slices of provolone and 4 oz. of sliced steak. Pour 4 oz. of gravy evenly over steak. Top with roughly 1/4 cup of arugula and top half of baguettes. Slice sandwiches in half and serve.
