In a medium bowl, whisk together olive oil, lemon juice, minced garlic, oregano, salt, and black pepper until fully emulsified.

Place flank steak in a large resealable bag or shallow dish. Pour marinade over steak, massage to coat evenly, seal and refrigerate for at least 30 minutes up to 2 hours.

Cook quinoa or couscous according to package instructions using water or chicken broth. Fluff with a fork and set aside.

Heat grill or grill pan over medium-high heat. Remove steak from marinade, letting excess drip off.

Grill steak for 4-5 minutes per side for medium-rare, or until desired doneness is reached.

Remove steak from heat and let rest for 5-10 minutes, then slice thinly against the grain.

In a large bowl, combine cherry tomatoes, cucumber, red onion, and olives. Mix gently.

Assemble bowls by adding cooked grain as base, topped with sliced steak, vegetable mixture, crumbled feta, and dollop of tzatziki sauce.

Garnish with fresh parsley or mint if desired and serve immediately.