Preheat the oven to 180°C (350°F) Gas Mark 4. Cut the onion in half, then peel and finely chop. Peel and trim the carrots and celery and then cut into dice.
Add the oil into a heavy-based casserole. Season the chicken fillets, then add to the casserole, skin side down and cook for 2-3 minutes until the skin has crisped up and browned. Using a tongs, transfer to a plate.
Add the onion and bacon to the pan and sauté for 2-3 minutes, then add the carrots, celery and thyme and cook for another 4-5 minutes. Stir in the rice and then pour in the stock and tomatoes. Season and arrange the chicken on top. Cover and bake for 20-25 minutes until the chicken is tender and all the rice has been absorbed.
Strip the parsley off the stems and roughly chop the leaves. Scatter over the baked chicken and bacon rice, then leave to stand for 5 minutes. Remove the lid and gently fluff up the rice with a fork. Serve straight to the table.