Remove the top sirloin steak from the refrigerator 20 minutes before cooking to reach room temperature.
Season both sides of the steak evenly with the sea salt and black pepper.
Heat the avocado oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Carefully place the steak in the hot skillet and sear for 4 minutes per side for a perfect medium-rare finish.
Transfer the steak to a clean cutting board and allow it to rest for 10 minutes to lock in the juices.
While the steak rests, whisk together the extra virgin olive oil, balsamic vinegar, and Dijon mustard in a small jar.
In a large bowl, combine the mixed greens, halved cherry tomatoes, sliced cucumber, and thinly sliced red onion.
Drizzle the dressing over the vegetables and toss gently to ensure everything is lightly coated.
Slice the rested steak against the grain and arrange it over the salad, finishing with a sprinkle of blue cheese.