Slice the ribeye steak into thin slices, so that the sandwich is easy to eat when assembled.
Heat the pan that was used for the onions, and add a little more olive oil if necessary. As you will want to pan roast the beef strips, you will need to heat this up on a high flame, without burning the oil.
Once hot, add the thinly sliced steak, season with a little salt and black pepper, and allow it to roast for a couple of minutes, then turn and do the same. You can cook them longer or shorter depending on your preferred internal temperature, however, the hot oil will offer an effect similar to a charcoal grill, where the grill marks would be. Try not to lower the flame below medium high heat.
Once the steak strips are ready, set them aside for a few minutes to rest on a cutting board, so that the juices of the meat can settle.
While the strips of steak rest, slice open the warm baguette or ciabatta rolls and add some raw tomato sauce, caramelized onions, and arugula [rocket] to them, then finally add the ribeye strips.
Serve immediately with your side of choice, like homemade baked potato wedges or a mixed salad.