In a large zip bag, place the chicken breasts, olive oil, lemon zest, lemon juice, garlic, oregano, salt, and pepper. Marinate in the refrigerator for at least 20 minutes, ideally 4 hours or overnight.
Cook the farro:
Rinse and drain farro. Place farro in a pot with salt and water or stock. Bring it to a boil, reduce heat to medium-low, and simmer for 30 minutes until the grain is tender. Drain off any excess water.
Cook the chicken:
In a large skillet, heat olive oil over medium-high heat, place the chicken breasts on the hot surface, and cook the first side for 7 minutes. Flip the chicken breast over and continue to cook for an additional 5-7 minutes until the internal temperature has reached 165F. Discard the marinade.
Remove chicken from pan onto a plate and wait 5 minutes before slicing.
Make the tzatziki sauce:
Grate the cucumber on top of a couple of paper towels. Once grated, squeeze any excess moisture over the sink and transfer the grated cucumber into a bowl. Grate the garlic clove in the bowl. Add yogurt, salt, lemon juice, and dill. Stir to combine and refrigerate until ready to pack.
Pack it for lunch:
In a lunchbox, make a base of farro. Top with sliced chicken, tomatoes, cucumber slices, olives, red onion, and feta cheese.
Pack the tzatziki sauce in a small leak-proof container and place it in the lunch box. Enjoy this lunch cold or at room temperature.