In a medium bowl, whisk together lime juice, olive oil, chopped cilantro, minced garlic, cumin, chili powder, salt, and black pepper until well combined. Place flank steak in a resealable plastic bag or shallow dish. Pour marinade over steak, coating fully. Seal and refrigerate for at least 30 minutes, preferably 2 to 4 hours.

While steak marinates, prepare rice according to package directions. In a small saucepan over medium heat, warm black beans for about 5 minutes, stirring occasionally. Season with salt or extra cumin if desired. If using fresh corn, boil briefly or sauté until tender. Heat frozen corn as needed.

Preheat grill or grill pan to medium-high heat and lightly oil the grates. Remove steak from marinade, letting excess drip off. Discard remaining marinade. Place steak on hot grill and cook for 4 to 5 minutes per side for medium-rare, or until desired doneness (130°F internal temperature for medium-rare). Let steak rest on a cutting board for 5 to 10 minutes, then slice thinly against the grain.

To assemble bowls, divide cooked rice among four bowls (about 1/2 to 1 cup per serving). Top each with portions of black beans, corn, and halved cherry tomatoes. Lay sliced steak over rice and vegetables. Add avocado slices, diced red onion, and crumbled feta if using. Garnish with extra cilantro and serve with lime wedges for squeezing.