For salsa, place first 5 ingredients in a small bowl; stir in 1 teaspoon cumin and 1/4 teaspoon salt. Let stand until serving.
Sprinkle steak with the remaining cumin and salt. Grill, covered, over medium heat or broil 4 in. from heat until meat reaches desired doneness (for medium-rare, a thermometer should read 135°), 6-8 minutes. Let stand 5 minutes.
Meanwhile, in a skillet, heat oil over medium-high heat; saute onion until crisp-tender. Slice steak thinly across the grain; serve in tortillas with onion and salsa. If desired, serve with avocado and lime wedges.
Wear disposable gloves when cutting hot peppers; the oils can burn skin. Avoid touching your face.