Season the sirloin steak generously with sea salt and black pepper on both sides.
Heat the extra virgin olive oil in a cast-iron skillet over medium-high heat until it begins to shimmer.
Place the steak in the hot skillet and sear for 3 to 4 minutes per side for medium-rare, or until it reaches your desired level of doneness.
Remove the steak from the pan and let it rest on a cutting board for at least 5 minutes to lock in the juices.
While the steak rests, whisk together the balsamic vinegar and Dijon mustard in a small bowl to create the dressing.
In a large salad bowl, combine the mixed baby greens, sliced cucumber, and thinly sliced red onion.
Slice the rested steak against the grain into thin strips.
Toss the salad greens with half of the balsamic dressing, then top with the sliced steak, diced avocado, and pomegranate arils.
Drizzle the remaining dressing over the top and serve immediately.