Whisk together lime juice, olive oil, chopped cilantro, minced garlic, cumin, chili powder, salt, and black pepper in a medium bowl until the marinade is well blended. Add the flank steak, turn to coat all sides, cover, and refrigerate for 30 minutes minimum or up to 4 hours.

While the steak marinates, cook your rice according to package directions and set aside. Heat the black beans in a small saucepan over medium heat for about 5 minutes with a pinch of cumin. If using fresh corn, boil or saute for 3-4 minutes until tender. For frozen corn, microwave or heat in a pan until warmed through. If using canned corn, simply drain and rinse. Chop all the fresh vegetables.

Preheat your grill or grill pan to medium-high heat. Remove the steak from the marinade, letting excess drip off, and discard the used marinade. Place steak on the hot grill and cook for 4-5 minutes per side for medium-rare (130°F internal temperature), or longer for your preferred doneness.

Remove the steak from heat and let it rest for 5-10 minutes on a cutting board. This crucial step keeps all those flavorful juices inside the meat.

Slice the rested steak thinly against the grain for maximum tenderness. Divide rice among four bowls, then add portions of black beans, corn, tomatoes, avocado, and red onion. Top with sliced steak, crumbled feta if using, fresh cilantro, and serve with lime wedges.