Heat the oil and butter in a frying pan until melted, then add the chicken, in batches if necessary, and cook until browned on all sides. Remove with a slotted spoon, leaving the fat in the pan, and transfer to the ceramic cooking pot. Switch the slow cooker to high.
Add the spring onions, garlic and chilli to the frying pan and cook gently for 4-5 minutes, or until softened, stirring frequently. Stir in the allspice, cinnamon, thyme, nutmeg and sugar. Sprinkle in the flour and stir to mix, then gradually add the chicken stock, stirring until the mixture bubbles and thickens. Remove the pan from the heat.
Stir the vinegar, lime juice, tomato paste and some salt and ground black pepper into the sauce. Pour over the chicken pieces, cover with a lid and cook on high for 3-4 hours, or until the chickeen is cooked and very tender.
Remove the chicken from the sauce and place on a serving dish. Taste the sauce and adjust the seasoning, then serve separately, with salad leaves or rice as an accompaniment. I usually chop up some tomatoes and peppers and wrap it all up in a tortilla.