In a medium bowl, combine the olive oil, lemon juice, minced garlic, oregano, salt, and pepper. Add the chicken cubes and toss until they are fully coated. Let the chicken marinate for at least 30 minutes.

While the chicken marinates, cook the quinoa. In a medium saucepan, bring the water or chicken broth to a boil. Add the rinsed quinoa, reduce the heat to low, cover, and simmer for about 15 minutes, or until the liquid is absorbed. Fluff the quinoa with a fork and set it aside.

Heat a large skillet or grill pan over medium-high heat. Add the marinated chicken and cook for 6-8 minutes, turning occasionally, until it's golden brown and cooked through.

In a small bowl, mix together the Greek yogurt, grated cucumber, lemon juice, fresh dill, minced garlic, and salt. Stir until well combined.

Divide the cooked quinoa among four bowls. Top each bowl with the cooked chicken, diced cucumber, cherry tomatoes, red onion, Kalamata olives, and crumbled feta cheese. Drizzle generously with the homemade tzatziki sauce and serve immediately.