In a medium bowl, whisk together the olive oil, lemon juice, 3 cloves of minced garlic, oregano, dried dill, salt, and pepper. Add the cubed chicken and toss until it's fully coated. Cover and refrigerate for at least 30 minutes, or up to 4 hours.

While the chicken is marinating, cook the quinoa according to the package instructions. Typically, this involves simmering 1 cup of quinoa with 2 cups of water for about 15 minutes. Once cooked, fluff with a fork and set aside.

Place the grated cucumber in a clean kitchen towel or paper towels and squeeze firmly to remove as much excess water as possible. In a small bowl, combine the squeezed cucumber, Greek yogurt, 1 tbsp lemon juice, 1 clove minced garlic, fresh dill, and 1/4 tsp salt. Stir until well combined. Refrigerate until ready to use.

Heat a large skillet or grill pan over medium-high heat. Add the marinated chicken in a single layer. Cook for 6-8 minutes, turning occasionally, until the chicken is golden brown and cooked through (internal temperature reaches 165°F or 74°C). Remove from skillet and let it rest for 5 minutes before dicing.

To build your Greek Chicken Bowls, divide the cooked quinoa evenly among four bowls. Top with the chopped romaine lettuce, diced chicken, cherry tomatoes, diced cucumber, sliced red onion, Kalamata olives, and crumbled feta cheese.

Drizzle each bowl generously with the homemade tzatziki sauce and serve immediately. Enjoy!