In a bowl, mix together olive oil, lemon juice, garlic, oregano, salt, and pepper. Add the chicken breasts and coat them evenly. Let it marinate for at least 30 minutes (or up to overnight) in the refrigerator.
Rinse the quinoa under cold water. In a medium saucepan, combine the quinoa and broth. Bring to a boil, then lower the heat, cover, and simmer for 15 minutes, or until the quinoa is tender and the liquid is absorbed. Fluff with a fork.
Heat a grill or skillet over medium-high heat. Cook the marinated chicken breasts for 6-7 minutes on each side until golden brown and the internal temperature reaches 165°F (75°C). Let the chicken rest for a few minutes, then slice into thin strips.
While the chicken cooks, dice the cucumber, halve the cherry tomatoes, and slice the red onion. Place them in a large bowl along with the Kalamata olives and parsley. Drizzle with a little olive oil and season with salt and pepper.
In a small bowl, combine the Greek yogurt, grated cucumber, dill, olive oil, and lemon juice. Season with salt and pepper to taste. Mix well until smooth.
In serving bowls, layer the cooked quinoa, sliced chicken, and veggie mixture. Sprinkle with crumbled feta cheese and drizzle with tzatziki sauce. Serve immediately, or refrigerate for later!
