Combine olive oil, minced garlic, ground cumin, chili powder, salt, and pepper to make the marinade. Rub over the flank steak and marinate for at least 30 minutes.

In a saucepan, bring water to a boil, then stir in the rice. Cover and simmer for 18-20 minutes until tender.

Preheat grill to medium-high. Grill marinated steak for 4-5 minutes per side until medium-rare. Let it rest for 5 minutes.

In a bowl, mix the cooked rice, black beans, corn, diced tomatoes, red onion, cilantro, and lime juice. Toss gently.

Slice the steak and serve over the rice mixture. Top with avocado, shredded cheese, and optional sour cream or salsa.