In a small bowl, mix together olive oil, chili powder, cumin, garlic powder, salt, pepper, and lime juice. Pour this marinade over the flank steak and let it marinate for at least 30 minutes (or up to 2 hours in the refrigerator for more flavor).
Preheat a grill or grill pan over medium-high heat. Remove the steak from the marinade and grill for about 4-5 minutes per side for medium-rare, or until desired doneness. Let the steak rest for 5 minutes before slicing it thinly against the grain.
While the steak is resting, prepare the bowls. In each bowl, layer 1/2 cup of cooked brown rice, 1/4 cup black beans, 1/4 cup corn, and a handful of cherry tomatoes.
Top each bowl with sliced steak, diced avocado, shredded cheese, and chopped cilantro. Serve with lime wedges on the side for extra flavor.