In a medium bowl, whisk together the olive oil, lemon juice, minced garlic, dried oregano, salt, and pepper. Add the cubed chicken and toss to coat completely. Cover and refrigerate for at least 30 minutes, or up to 4 hours.

While the chicken marinates, cook the quinoa. Add the rinsed quinoa and water (or broth) to a medium saucepan. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes, or until all the liquid is absorbed. Remove from heat and let it stand, covered, for 5 minutes. Fluff with a fork.

Prepare the tzatziki: In a small bowl, combine the Greek yogurt, grated and squeezed cucumber, minced garlic, fresh dill, lemon juice, and salt. Stir well and refrigerate until ready to use.

Prepare the cucumber tomato salad: In another bowl, gently toss together the halved cherry tomatoes, diced cucumber, sliced red onion, and chopped parsley. Drizzle with olive oil and lemon juice, and toss to combine. Set aside.

Heat a large skillet or grill pan over medium-high heat. Add the marinated chicken in a single layer (you may need to cook in batches). Cook for 6-8 minutes, turning occasionally, until golden brown and cooked through (internal temperature reaches 165°F or 74°C).

Divide the cooked quinoa among four bowls. Top with the cooked chicken, a generous scoop of the cucumber tomato salad, and a dollop of tzatziki sauce. Garnish with crumbled feta cheese and Kalamata olives. Serve immediately.