In a mixing bowl, combine lime juice, chili powder, garlic powder, olive oil, and salt to create the marinade. Place flank steak in a resealable bag or shallow dish, pour marinade over it, seal, and refrigerate for at least 2 hours, or overnight.

Prepare the salsa by dicing avocado and chopping jalapeño in a separate bowl. Add chopped cilantro, season with lime juice and salt, mix gently, and set aside.

Preheat the grill to medium-high heat (around 400°F). Take marinated steak out of the fridge and let it sit at room temperature for 15 minutes.

Grill steak for 6–8 minutes on each side for medium-rare, flipping only once, until internal temperature reaches 135°F.

Transfer the steak to a cutting board and rest for 5 minutes, covered with foil.

Slice steak against the grain into thin strips and serve topped with avocado salsa.