HEAT a large cast iron pan over medium-high until very hot. Brush steak with 1/2 tbsp of the oil, then sprinkle with 1/2 tsp of the salt and 1/4 tsp of the pepper. Add to skillet and cook, 3 to 4 min per side for medium-rare. Transfer to a cutting board and cover loosely with foil for 5 min. Discard any fat from pan.
RETURN pan to heat over medium-high, then add corn and cook, stirring, until charred, 4 min.
PULSE remaining 2 tbsp oil, vinegar, remaining 1/2 tsp salt, and remaining 1/4 tsp pepper, cilantro, onion and garlic until almost smooth.
ARRANGE arugula on a platter. Cut steak into thin strips and place on arugula. Spoon corn and avocado over steak and drizzle with cilantro mixture.