Combine ¼ cup olive oil, juice of 2 limes, 4 minced garlic cloves, and a pinch of salt and pepper in a bowl. Add steak and coat well. Cover and refrigerate for at least 30 minutes, up to 2 hours.
In a food processor, combine 1 cup cilantro, ¼ cup olive oil, 2 tablespoons lime juice, 2 minced garlic cloves, and a pinch of salt. Blend until smooth. Adjust seasoning if necessary.
Preheat your grill to medium-high heat.
Remove steak from marinade, letting excess drip off. Grill for 4-5 minutes on each side for medium-rare, aiming for 130°F (54°C).
Transfer steak to a cutting board and let rest for 5-10 minutes.
Slice steak against the grain into thin strips.
Warm corn tortillas on the grill for about 30 seconds on each side.
Place steak slices on each tortilla, drizzle with chimichurri sauce, and top with avocado.
Serve immediately and enjoy!