Season the steak slices with salt and black pepper.
Heat the butter in a non-stick pan. Place the steak in the pan and cook until done. Let the steak rest for a few minutes before slicing it.
In a large bowl, combine the lettuce, arugula, avocado, cherry tomatoes, cucumber, and red onion. Add the sliced steak and sprinkle the cheese on top.
In a small bowl, whisk together the olive oil, lemon juice, salt, black pepper, and mustard.
Drizzle the dressing over the salad and toss gently. Serve the Keto Steak Salad immediately.