Combine jalapeno, garlic, cilantro, lime juice, oil, salt, pepper, and cumin in a large resealable bag or bowl. Add steak and toss to coat; if using bowl, wrap with plastic. Marinate at least 30 minutes at room temperature or chill up to overnight in refrigerator, tossing occasionally. Let steak sit at room temperature 30 minutes before cooking if chilled. Prepare a grill or grill pan for medium-high heat. Remove steak from marinade, scraping off any bits that cling to meat; discard marinade. Grill steak 23 minutes per side for medium rare. Let rest 10 minutes before thinly slicing against the grain. Warm tortillas in a microwave, oven, or over a gas flame. Divide steak among tortillas, then top with salsa, avocado, sour cream, queso fresco, if using, cilantro, and onion. Serve with lime wedges alongside. Steak can be marinated 1 day ahead; keep chilled.