If marinating the steak, place it in a casserole dish. Pour the Steak Marinade over the steak and turn to coat completely. Cover and refrigerate for a few hours or overnight.
When ready to cook, remove the steak from the marinade, if using, and allow the steak to come to room temperature. Preheat a grill pan or outdoor grill to high. Season the steak with salt and black pepper. Grill the steak to the desired doneness, 10 to 15 minutes for medium rare (an instant-read thermometer inserted into the thickest part of the steak should register 130 degrees F). Place the meat on a cutting board and allow to rest at least 5 minutes.
While the steak cooks and rests, slice the onions thinly into half moons. Slice the mushrooms very thinly. Heat a large skillet over high heat. Add 1/2 cup of oil. Once the oil is hot, add the onions and cook, stirring frequently, until lightly caramelized. Add the mushrooms. Turn the heat down to medium and continue to cook until the liquid from the mushrooms has evaporated, about 10 minutes. Season with salt and pepper.
Drizzle a little oil on the cut sides of the ciabatta and toast on the grill.
Stir the mayonnaise and mustard together in a small bowl. Set aside.
To assemble the sandwiches, thinly slice the steak against the grain. Place a dollop of the mayonnaise mixture on both halves of the toasted ciabatta loaf or rolls and spread evenly. Layer the sliced Gruyere on the top half of the ciabatta. (If you want melted cheese, place the Gruyere-topped ciabatta under a preheated broiler for a minute or two.) Layer the sliced steak on the bottom half of bread. Next, top the steak with the mushrooms and onions. Close the sandwich with the top half of ciabatta, slice and serve.
Steak Marinade (optional):
Combine the Worcestershire, oil, garlic, rosemary and chile in a large bowl and mix well.