Heat 2 tablespoons of the olive oil in a large sauté pan over medium-high heat. When hot, add the cremini mushrooms and cook until golden brown, about 2 minutes. Turn the mushrooms over and cook for another 2 minutes, and then season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Remove the mushrooms from the pan and set aside.

Heat a grill to medium-high heat. Brush the steaks with 1 tablespoon olive oil and season with 1 teaspoon salt and 1/2 teaspoon pepper. Grill the steaks for 4 minutes per side for rare to medium-rare. Remove the steaks from the grill and let them rest, lightly covered with aluminum foil, for at least 5 minutes. Slice the steak into thin strips and set aside.

In a small bowl, combine the mayonnaise and the prepared horeradish and season with a pinch of salt and pepper.

Slice the ciabatta rolls in half horizontally and toast lightly.

Spread 1 tablespoon of the horseradish mayonnaise on each cut side of the rolls. Layer the bottom of each roll with 6 ounces of sliced steak, 3 tablespoons of the sauteed mushrooms, 2 tablespoons of the Caramelized Onions and several leaves of the spinach. Close the sandwich with the top portion of the roll and then slice each sandwich in half and serve with a small side salad or potato chips.