Marinate the Steak: 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 at least 30 minutes, preferably 2-4 hours.
Prepare Rice and Beans: 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 (boil or sauté briefly). Heat frozen corn as needed.
Cook the Steak: Preheat grill or grill pan to medium-high.
Remove steak from marinade and let excess drip off. Discard marinade.
Grill steak 4-5 minutes per side for medium-rare or until desired doneness (130°F for medium-rare).
Let rest 5-10 minutes, then slice thinly against the grain.
Assemble Bowls: 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!