Prepare the steaks: Pat the ribeye steaks dry with paper towels and season generously with salt and freshly ground black pepper.

Cook the steaks: Heat a large cast-iron pan over medium-high heat and add the vegetable oil. Once heated, add the steaks and cook for about 3 minutes, or until the bottom develops a golden-brown crust. Flip the steaks, add the butter to the pan, and baste the steaks with the melted butter. Continue cooking for an additional 3 minutes, or until medium-rare (internal temperature should be 120°F / 49°C). Remove the steaks from the pan and let them rest for 10 minutes before slicing thinly.

Make the au poivre sauce: In the same pan, add the diced shallot and sauté for 2 minutes until tender and opaque. Add the cracked peppercorns and sauté for another minute. Add the cognac and cook for about 1 minute to burn off the alcohol. Pour in the beef stock and simmer until the sauce is reduced by half, about 4 minutes. Whisk in the cream and cook until the sauce coats the back of a spoon, about 2 minutes. Set the sauce aside.

Prepare the arugula salad: In a medium bowl, toss the arugula with olive oil and red wine vinegar.

Assemble the sandwich: Divide the sliced steak evenly between the bottom halves of the toasted ciabatta bread. Top each portion with the au poivre sauce, followed by a handful of the arugula salad and a spoonful of pickled red onions. Close the sandwich, cut in half, and serve immediately.