Marinate the chicken: In a bowl, whisk olive oil, lemon juice and zest, garlic, oregano, cumin, salt, and pepper. Add the chicken and toss to coat. Marinate for at least 20 minutes, or up to 8 hours in the fridge.
Cook the grains: While the chicken marinates, cook quinoa, farro, or brown rice according to package directions.
Fluff and set aside to cool slightly.
Make the tzatziki: Grate the cucumber and squeeze out excess water with a clean towel. Stir together yogurt, cucumber, lemon juice, olive oil, garlic, dill, salt, and pepper. Taste and adjust seasoning.
Chill until serving.
Prep the veggies: Slice tomatoes, chop cucumbers, thinly slice red onion, and halve olives. Rinse and chop greens.
Cook the chicken: Heat a skillet or grill pan over medium-high. Lightly oil the surface.
Cook chicken 5–7 minutes per side (thighs) or 4–6 minutes per side (breasts), until browned and cooked through. Rest 5 minutes, then slice.
Assemble the bowls: Divide grains among bowls. Top with greens, tomatoes, cucumbers, onion, olives, and sliced chicken.
Add a generous dollop of tzatziki and sprinkle with feta and herbs. Finish with a squeeze of lemon and a drizzle of olive oil.
Serve: Season with a pinch of salt and pepper if needed. Enjoy warm or at room temperature.