Follow the instructions in my Pan Seared Steak tutorial to cook the steaks to your desired doneness.
Make the caramelized onions (recipe link in the ingredients list) and sautéed mushrooms - I typically make these while the horseradish mayo (below) is resting.
For the mushrooms, heat the butter in a nonstick skillet over medium-high heat, until foaming subsides. Add the mushrooms, along with a pinch of kosher salt and pepper. Sauté until the liquid is released and evaporated and the mushrooms are beginning to brown around the edges. Remove from the pan and set aside.
Make the Horseradish Mayo
In a bowl, whisk together mayonnaise, sour cream, garlic, horseradish, and a pinch or two of salt and pepper. Cover and refrigerate for at least 30 minutes for the flavors to meld. You will not taste the full flavor of the horseradish when you first mix the mayo.
After 30 minutes, taste the mayo and add additional horseradish to your preferences. Keep chilled until ready to assemble the sandwiches.
Assemble the Sandwiches
Spread a little bit of the mayo on the bottom of each ciabatta roll. Layer each with a few baby arugula leaves, ¼ of the sliced steak, sautéed mushrooms, and caramelized onions. If desired, spread a little extra horseradish mayo on the top half of the roll.
Sandwiches are best served immediately, while the steak is still warm.