Heat the oil in a large saucepan over a medium heat.
Add the onion, celery, and carrots to the pan and fry for 4-5 minutes, until just starting to soften.
Add the flour to the pan and stir together using a whisk to coat the vegetables in the flour.
Pour in the milk, slowly, whilst stirring with the whisk, until the milk is completely mixed in.
Add the stock and dried thyme and stir again.
Add the rice, bring to the boil, then turn down the heat and simmer for 25 minutes – stirring occasionally, until the rice is tender.
Add in the chicken, lemon juice, cream, parmesan, salt, and pepper.
Stir together and heat for 2-3 minutes until the chicken is warmed through.
Serve topped with fresh parsley.
