First prepare the stuffing. Heat a large frying pan over medium heat and add a glug of oil. Sauté the onion for 4 minutes, then add the garlic and continue to cook for another 1–2 minutes until softened. Add the celery, then stir in the pine nuts and chestnuts, taste, and season well with salt and pepper. Add the cooked rice and parsley, stir again, then taste and adjust the seasoning as necessary.