Grate English cucumber onto a kitchen towel and squeeze firmly to remove all excess liquid. In a medium bowl, combine the squeezed cucumber, Greek yogurt, minced garlic, lemon juice, 1 tsp of dried dill, and salt. Stir until combined. Cover and refrigerate.

In a large bowl, combine chicken chunks with olive oil, lemon juice, dried oregano, garlic powder, salt, and pepper. Preheat grill or grill pan over medium-high heat. Grill chicken for 4-6 minutes per side, until charred and cooked through to 165°F (74°C). Let rest for 5 minutes.

Thoroughly rinse uncooked quinoa. In a saucepan, combine quinoa, water or broth, and a pinch of salt. Bring to a boil, then cover and simmer on low for 15 minutes, or until liquid is absorbed. Remove from heat, let stand covered for 5 minutes, then fluff with a fork.

Divide quinoa among four bowls. Top with chicken, diced cucumbers, halved cherry tomatoes, kalamata olives, and sliced red onion, arranging them in distinct sections. Sprinkle with feta cheese, add a dollop of tzatziki in the center, and garnish the sauce with a pinch of dry dill flakes.