In a large bowl, combine 3 tbsp olive oil, 1/4 cup lime juice, 1/4 cup chopped fresh cilantro, 3 cloves minced garlic, 1 tsp cumin, 1/2 tsp chili powder, 1 tsp salt, and 1/2 tsp black pepper. Add the flank steak, ensuring it's fully coated in the marinade. Let it marinate at room temperature for at least 30 minutes, or refrigerate for up to 2 hours for deeper flavor.

Rinse 1 cup of basmati or long-grain rice thoroughly. In a medium saucepan, combine the rinsed rice, 2 cups water or chicken broth, and 1/2 tsp salt. Bring to a boil over medium-high heat, then reduce heat to low, cover, and simmer for 15-18 minutes, or until all the liquid is absorbed. Remove from heat and let stand, covered, for 5 minutes. Fluff with a fork, then stir in 2 tbsp lime juice and 1/4 cup fresh chopped cilantro.

While the rice cooks and steak marinates, prepare your toppings: dice the avocados, thinly slice or dice the red onion, rinse and drain the black beans, and thaw or drain the corn.

Heat a cast-iron skillet or grill pan over medium-high heat until it's very hot. Remove the steak from the marinade, discarding any excess marinade. Sear the steak for 4-6 minutes per side for medium-rare, or adjust cooking time to your desired level of doneness.

Transfer the cooked steak to a cutting board and let it rest for 5-10 minutes. This is crucial for keeping the steak juicy. After resting, slice the steak thinly against the grain.

Divide the fluffy cilantro-lime rice among 4 serving bowls. Top each bowl generously with the sliced steak, black beans, corn, diced avocado, sliced red onion, a spoonful of salsa, and a dollop of sour cream or Greek yogurt. Garnish with fresh lime wedges and extra cilantro if desired. Serve immediately.