Sprinkle steak with salt and pepper. Grill steak, covered, over medium heat or broil 4 in. from heat until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 160°), 6-9 minutes per side. Let stand 5 minutes.
Meanwhile, process salsa and cilantro in a food processor until blended. Slice steak thinly across the grain; serve with salsa mixture, avocado and tomato.