Juice one lemon and thinly slice another. Reserve the slices for serving. In a bowl, mix together the half lemon juice, 1 tablespoon of olive oil, garlic, ground oregano, salt, and ground white pepper. Add the chicken thighs and marinate for 10 minutes.
Heat 1 tablespoon of olive oil in a frying pan over medium-high heat. Place the chicken thighs in the pan and cook for 4 minutes on each side until they are golden brown and half-cooked. Transfer to a plate.
Meanwhile, make the Greek yogurt spinach sauce in a food processor. Blend together the remaining lemon juice, baby spinach, basil leaves, chicken stock, and Greek yogurt for 30 seconds until smooth.
Heat the remaining 1 tablespoon of oil in a frying pan over medium heat. Add the onion and stir-fry until aromatic. Stir in the rice, then add the yogurt spinach mixture and bring to a simmer. Return the chicken thighs and any juices to the pan, cover with a lid, reduce heat to medium-low, and cook for 15 minutes or until the rice is soft and the chicken is cooked through.
Turn off the heat, leave the lid covered for another 5 minutes, then serve with lemon slices and feta cheese crumbles on top.