In a small bowl, whisk together olive oil, lime juice, garlic, cumin, chili powder, salt, and pepper. Place the flank steak in a resealable plastic bag or shallow dish and pour the marinade over it. Seal the bag or cover the dish and refrigerate for at least 30 minutes, or up to 4 hours for more flavor.
Preheat a grill or grill pan over medium-high heat. Remove the steak from the marinade and discard the marinade. Grill the steak for about 5-7 minutes per side, or until it reaches your desired level of doneness. Let the steak rest for 5 minutes before slicing it thinly against the grain.
In a large bowl, combine the cooked brown rice, black beans, corn, cherry tomatoes, and half of the cilantro. Toss gently to mix.
To assemble the bowls, divide the rice mixture among four bowls. Top each bowl with sliced steak, avocado, and the remaining cilantro. Serve with lime wedges on the side.