Remove the steak from the fridge, remove packaging, and set on a cutting board to come to room temperature while you prepare the vinaigrette and dry rub.
Place all vinaigrette ingredients into a blender, and blend until smooth. Set aside.
Prepare the fajita dry rub. Place all dry rub ingredients into a small bowl, then mix until thoroughly combined. Rub the fajita seasoning into both sides of the steak.
Heat a 12" cast iron skillet or grill pan over high heat. Add the steak to the grill pan and sear 3-4 minutes per side for medium rare. Remove the steak from the skillet and place on a cutting board to rest while you cook the peppers and onions.
Reduce the head to medium high, then add the oil to the skillet. Add the peppers and onions and toss to coat in the fat and leftover fajita spices. Saute the vegetables, tossing frequently, until soft and charred in spots, 6-7 minutes. Remove the skillet from heat.
Thinly slice the steak against the grain.
Prepare the salad. Divide the lettuce between four bowls, then top with half a sliced avocado and one-quarter of the tomatoes. Top each salad bowl with a quarter of the onions and peppers and a quarter of the steak. Sprinkle with cheese if using, and drizzle with cilantro lime vinaigrette to taste. Serve immediately.