In a large mixing bowl, combine 1/4 cup olive oil, 2 tablespoons fresh lemon juice, dried oregano, garlic powder, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Whisk until well combined.

Add the cubed chicken breasts to the marinade. Toss to coat evenly. Cover the bowl and refrigerate for at least 20 minutes, or up to 2 hours for best flavor.

While the chicken marinates, combine the rinsed quinoa, water (or broth), and 1/2 teaspoon salt in a medium saucepan. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes, or until all liquid is absorbed and quinoa is fluffy. Remove from heat and let stand, covered, for 5 minutes, then fluff with a fork.

In a small bowl, combine Greek yogurt, grated and squeezed dry cucumber, minced garlic, 1 tablespoon fresh lemon juice, fresh dill, 1/4 teaspoon salt, and a pinch of black pepper. Stir well and refrigerate until ready to serve.

In a separate medium bowl, combine the halved cherry tomatoes, diced cucumber, thinly sliced red onion, halved Kalamata olives, and crumbled feta cheese. Set aside.

Heat a large skillet or grill pan over medium-high heat. Add the marinated chicken in a single layer (cook in batches if necessary to avoid overcrowding). Cook for 5-7 minutes, turning occasionally, until golden brown and cooked through (internal temperature reaches 165°F / 74°C).

Divide the cooked quinoa evenly among 4 serving bowls. Top each bowl generously with the cooked Greek chicken.

Spoon the prepared Greek salad topping over the chicken and quinoa. Dollop a generous amount of tzatziki on top. Garnish with a fresh lemon wedge and serve immediately.