Add the pancetta to a large deep heavy-based pot over medium heat (no oil needed). Fry until the pancetta is crisp with the fat rendered down. Remove and place in a large baking dish (mine is 12x9" / 30x22cm), leaving the fat behind.
Season the chicken thighs with 1/2 tsp salt and pepper. Increase the heat to high and fry the thighs for around 1 minute 30 seconds each side, or until light golden. Don't try and cook the chicken through at this point, this is just to add some colour and develop flavour. Work in batches if you need to.
Remove the thighs and place in the baking dish with the pancetta, leaving all of the excess fat behind. Lower the heat slightly to medium-high and add the carrot, celery and onion. Fry until it all softens and begins to colour, then add the garlic and fry for another minute. Stir in the tomato puree and fry for a minute, then stir in the wine. Simmer for 2 minutes to reduce and re-thicken into a paste.
Pour in the passata and swill out the jar with 1/4 cup of water. Stir in the chicken stock, bay leaves and thyme, then stir in the chicken thighs pancetta alongside all of the resting juices. Bring to a simmer, then place the lid on, reduce heat to low and allow to gently bubble away for 30 minutes. Remove the lid and gently simmer for a further 20 minutes to thicken the sauce.
Use tongs to remove the chicken and place it back in the baking dish. Shred the chicken with two forks, then stir it back through the sauce. Adjust seasoning if desired then leave to one side as you proceed with the bechamel sauce.