In a bowl, whisk together lime juice, olive oil, chopped cilantro, minced garlic, cumin, chili powder, salt, and pepper. Reserve a small amount of marinade for drizzling later. Add the flank steak to the marinade, ensuring it's coated well. Cover and let marinate in the refrigerator for at least 30 minutes, or up to 4 hours for more flavor.
While the steak is marinating, rinse 1/2 cup of dry quinoa under cold water. In a saucepan, combine quinoa with 1 cup of water. Bring to a boil, then reduce to a simmer, cover, and cook for about 15 minutes until the water is absorbed. Fluff with a fork and set aside.
Preheat the grill or a skillet over medium-high heat. Remove the steak from the marinade and grill for about 5-7 minutes on each side, or until it reaches your desired degree of doneness. Let it rest for 5 minutes before slicing against the grain.
In each serving bowl, start with a base of quinoa. Top with sliced grilled steak, halved cherry tomatoes, avocado slices, corn, and red onion.
Drizzle the reserved marinade over the top and sprinkle with crumbled feta cheese if desired. Garnish with extra cilantro for a fresh finish.
Notes
Marinate the steak for longer for enhanced flavor.