In a medium bowl, whisk together lime juice, olive oil, cilantro, garlic, cumin, chili powder, salt, and black pepper. Add flank steak and coat well. Cover and refrigerate for at least 30 minutes, preferably 2 to 4 hours.
Cook rice according to package directions. Heat black beans in a small saucepan over medium heat for about 5 minutes. Season with a pinch of salt or cumin if desired. Cook corn if using fresh by boiling or sautéing briefly. Heat frozen corn as needed.
Preheat grill or grill pan to medium-high. Remove steak from marinade and let excess drip off. Discard marinade. Grill steak for 4 to 5 minutes per side for medium rare or until desired doneness (130°F for medium rare). Let rest for 5 to 10 minutes, then slice thinly against the grain.
Divide rice into bowls. Add black beans, corn, cherry tomatoes, avocado slices, and red onion. Top with sliced steak and feta if using. Garnish with extra cilantro and lime wedges. Enjoy!
Notes
Let the steak rest after grilling to retain juices. Use a good sear for flavor.