Place a large pot over medium heat and add the olive oil. Add the onion, carrots, celery, garlic and thyme . Cook and stir until the vegetables are softened but not browned, about 5 minutes.
Pour in the chicken stock/broth and bring the soup to a simmer. Add in the rice and chicken; season with salt and pepper.
Cook over medium-low heat until the rice is tender, about 25-30 minutes. Add lemon juice and zest just before serving. Serve warm.