Heat a grill to medium heat.
Remove steaks from the fridge and rub with olive oil. Season with steak seasoning to taste. Set aside on the counter while you prepare the onions and mushrooms.
Tear off a 16-inch sheet of Reynolds Wrap® Heavy Duty Foil and spray with cooking spray.
Place onion, mushrooms, 2 tablespoons garlic butter and herbs in the center of the foil sheet. Season with salt and pepper.
Bring up the long sides of the foil, so the ends meet and double-fold the ends. Double-fold the two short ends to seal the packet.
Place packet on the preheated grill and grill 10–15 minutes on one side. Flip package over and grill an additional 15 minutes or until onion is tender.
Add the steaks to the grill (after the initial 15 minutes of cooking the foil packets) and cook 5–6 minutes. Flip steaks and cook an additional 6–8 minutes or until it reaches 140 F or desired doneness.
Remove steak from the grill and place on a plate. Loosely tent with Reynolds Wrap® Aluminum Foil and let rest 5–7 minutes before slicing.
While steak is resting, butter the ciabatta rolls with garlic butter. Place butter side down on the grill and grill 3–4 minutes or until lightly browned.
Slice steak across the grain into 1/2-inch slices.
Spread each toasted roll with Dijon mustard (if using) and a small handful of arugula. Add sliced steak, mushrooms and onions (and spoon any butter in the packet over the steaks). Serve immediately.