Mix all the dry rub ingredients in a large bowl, add the chicken thighs and coat well. Cover the bowl and place in the fridge for a minimum of 2 hours, overnight is best if you have the time.
Set the slow cooker to high. Place a large frying pan over a medium heat and add the olive oil.
When the pan is hot add the onions and gently fry for around 5 minutes, until translucent and slightly golden. Tip the onions into the slow cooker and place the pan back on the heat.
Allow the pan to warm up again, then add the chicken and any extra rub. Seal on both sides for a minute and tip into to the slow cooker.
Place the pan back on the heat, add the tomato puree and mustard and stir for 30 seconds. Pour in the vinegar and Worcestershire sauce and stir for another 30 seconds. Finally add the chicken stock and scrape any brown/sticky bits from the bottom and sides of the pan and tip into the slow cooker.
Stir the chopped tomatoes and herbs into the slow cooker, pop the lid on and turn the slow cooker down to low. Cook for 4 hours, until the chicken is cooked and tender. Check in on your slow cooker and stir occasionally. Check your slow cooker manual as timings can differ.
When the chicken is ready remove it from the slow cooker using a slotted spoon and place in a serving bowl. Remove and discard the bay leaves and thyme stalks. Use a hand blender to blitz the juices into a smooth sauce. If the sauce is a bit runny you can boil in a pan for 5-10 minutes to thicken.
Tip the baked beans into a pan and add a ladle of the sauce. Cook on a low heat until piping hot and serve with the chicken.
Use two forks to shred the chicken. Mix some sauce back into the shredded chicken and serve with soft rolls, coleslaw, wedges and some leftover sauce on the side.