Melt half the butter in a large pan over medium heat. Cook the chicken until brown, about 5 minutes per side; transfer to a large casserole pot.
Add the remaining butter to the pan; cook the onion, celery and mushrooms until tender, about 5 minutes. Dust with flour and continue cooking for 3 minutes. Stir in the cider, stock and tomato puree; add the bay leaf, parsley and thyme. Bring to the boil until slightly reduced and the sauce coats the back of a spoon. Season with salt and pepper and pour sauce over chicken.
Cook in the preheated oven until the chicken is cooked through and the sauce is bubbly, 40 to 45 minutes.
Had this tonight for dinner. YUMMY!! Cooked it through the night before and dumped it in the slow cooker to warm through while waiting for hubby to finish work. This didn't spoil or overcook it at all. I didn't have fresh thyme and so compromised with dried but wont do that again as it made it a little too herby. All in all, an easy, hearty dinner for the whole family! - 03 Jul 2013