If using a leftover chicken carcass, place in a large saucepan and cover with 2 litres of water. Bring to the boil, then cover with a lid and gently simmer for 20 mins. Meanwhile, heat the oil in another large saucepan. If using chicken thighs, add these and brown on all sides, then remove and set aside.
Add the onions to the oil and cook for 10 mins until starting to caramelise. Add the carrots, herbs, spices and flour, and stir for 1-2 mins to toast the spices. If using a chicken carcass, strain the cooking liquid into the pan with the vegetables, otherwise add the browned chicken thighs and stock. Stir well, cover and simmer for 30 mins.
If using chicken thighs, remove them from the pan, shred the meat and discard the bones. Add the meat (or the meat from the carcass) back into the soup, along with the butter beans, season and heat through for 1-2 mins.
Use a hand-held blender to blitz the soup until smooth, or leave it chunky. Or blend about half, so it’s creamy but still has chunks of chicken, carrot and butter bean. Serve with extra pepper, and good crusty bread.