Marinate the chicken: In a bowl, whisk lemon juice, zest, olive oil, garlic, oregano, thyme, cumin, salt, and pepper. Add chicken and coat well. Marinate at least 20 minutes, or up to 8 hours in the fridge.
Make the tzatziki: Grate the cucumber, then squeeze out excess water with a clean towel.
Stir together yogurt, cucumber, lemon juice, garlic, dill, salt, and pepper. Chill to let flavors meld.
Prep the veggies: Dice cucumber, halve tomatoes, slice red onion, and dice bell pepper. Toss with a pinch of salt and a splash of lemon juice if you like extra brightness.
Cook your base: Prepare brown rice, quinoa, or cauliflower rice according to package directions.
Fluff and let cool slightly before portioning.
Cook the chicken: Heat a grill pan or skillet over medium-high. Cook chicken 4–6 minutes per side, until browned and cooked through (internal temp 165°F/74°C). Rest 5 minutes, then slice.
Assemble the bowls: Divide the grain or cauliflower rice among 4 containers.
Top with sliced chicken, cucumber, tomatoes, red onion, bell pepper, and olives. Sprinkle parsley or dill and a little feta if using.
Add tzatziki: Portion 2–3 tablespoons of tzatziki into small lidded sauce cups, or spoon on just before eating. Add lemon wedges to containers if desired.
Seal and store: Let everything cool fully before sealing to avoid condensation.
Store bowls and tzatziki separately for best texture.