If marinating the steak, place it in a casserole dish. Pour the Steak Marinade over the steak and turn to coat completely. Cover and marinate 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.
Meanwhile, slice the onions thinly into half moons. Heat a large skillet over high heat. Add 1/2 cup of olive oil. Once the oil is hot, add the onions and cook, stirring, until softened. Turn the heat down to medium and continue to cook the onions until they begin to caramelize, 5 to 8 minutes. Season with salt and pepper.
Combine the sun-dried tomatoes and prepared horseradish in a food processor and pulse to combine. In a large mixing bowl, stir together the fresh horseradish, mayonnaise and sour cream. Add the sun-dried tomato mixture and mix well. Season with salt and pepper. Set aside. (Can be made a day or 2 ahead.)
Drizzle a little oil on the cut sides of the ciabatta and toast on the grill.
To assemble the sandwiches, toss the arugula in a small bowl with a little olive oil and salt. Thinly slice the steak against the grain. Place a dollop of the sun-dried tomato mixture on both halves of the toasted ciabatta loaf or rolls. Layer the steak over the tomato. Next, top the steak with the caramelized onions and some arugula. 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.