Pat the chicken breasts dry with paper towels and place them in a large mixing bowl. Add the Greek yogurt, olive oil, fresh lemon juice, minced garlic, cumin, smoked paprika, coriander, turmeric, cinnamon, onion powder, salt, and black pepper. Mix thoroughly until every piece of chicken is evenly coated. Cover and refrigerate for at least 1 hour, or preferably 4–8 hours for deeper flavor and maximum tenderness.
Heat a large skillet or grill pan over medium-high heat. Place the marinated chicken into the hot pan, allowing excess marinade to remain on the surface. Cook for 5–7 minutes on the first side without moving it so a flavorful crust develops. Flip and cook for another 5–7 minutes, or until the thickest part reaches an internal temperature of 165°F (74°C). Transfer the chicken to a cutting board and let it rest for 5–10 minutes before slicing against the grain.
While the chicken is resting, cook the brown rice according to the package directions if it hasn't already been prepared. Fluff the rice with a fork and stir in a squeeze of fresh lemon juice and a little chopped parsley if desired.
Prepare the garlic yogurt sauce by whisking together the Greek yogurt, chopped dill, minced garlic, fresh lemon juice, tahini (if using), a pinch of salt, and black pepper until smooth and creamy. Refrigerate until ready to serve.
Wash and thoroughly dry the romaine lettuce, cucumber, cherry tomatoes, parsley, and red onion. Chop the lettuce, dice the cucumber, halve the tomatoes, and thinly slice the onion. Keep the vegetables chilled until assembly for the freshest texture.
Divide the cooked rice evenly among four serving bowls or meal prep containers. Arrange the sliced chicken over the rice, then add the romaine lettuce, cucumbers, tomatoes, and red onion in separate sections. Sprinkle with chopped parsley and optional crumbled feta cheese.
Drizzle the garlic yogurt sauce over the bowls just before serving or pack it separately if meal prepping. Finish each bowl with an extra squeeze of fresh lemon juice and additional parsley for the brightest flavor.
Serve immediately while the chicken is warm, or refrigerate the assembled bowls for up to four days. Reheat only the chicken and rice before adding the fresh vegetables and yogurt sauce if serving later.