Heat a cast iron skillet over high heat. Season the steak with the blackening seasoning on both sides. Add oil to the pan and sear the steak for 2 to 3 minutes on each side and continue to cook to your desired doneness. Set aside to rest, but keep the skillet on.
Toss the tomatoes with a drizzle of olive oil and salt and pepper. Blister the tomatoes in the cast iron pan until they are caramelized and start to burst open.
Assemble the salads (this amount should make 4 entree salads) by topping the romaine or spring mix with red onion, cucumber, avocado, and the blistered tomatoes. Slice the steak and divide it among the salads. Top with the blue cheese and fried onions. To dress, simply drizzle with olive oil and balsamic vinegar.