Marinate the chicken: In a bowl, whisk olive oil, lemon zest and juice, garlic, oregano, cumin, smoked paprika, salt, and pepper. Add chicken and toss to coat. Marinate at least 30 minutes or up to 12 hours in the fridge.
Cook the grains: Prepare quinoa or brown rice according to package directions.
Fluff with a fork and set aside to cool slightly.
Make the tzatziki: Stir yogurt, grated cucumber, lemon juice, garlic, dill, salt, and pepper. Adjust to taste. Refrigerate until serving.
Cook the chicken: Heat a skillet or grill pan over medium-high.
Add a light drizzle of oil. Cook chicken 5–7 minutes per side, until the thickest part reaches 165°F (74°C). Rest 5 minutes, then slice.
Prep the veggies: Dice cucumber, halve tomatoes, slice red onion, and chop parsley.
Rinse and drain chickpeas. Halve olives.
Assemble the bowls: Divide grains among 4 meal prep containers. Top with chicken, chickpeas, tomatoes, cucumber, onion, and olives.
Sprinkle with feta and parsley.
Add sauces and finishing touches: Add a small container of tzatziki to each bowl. For extra brightness, add a lemon wedge or a tiny drizzle of olive oil before eating.
Cool and store: Let everything cool to room temperature before sealing the containers to prevent condensation and sogginess.