Trim fat from the oxtail and separate the joints if necessary. Brown sufficient butter on a medium heat in pressure cooker. Fry the oxtail in the butter until the pieces are brown. Remove the pieces of oxtail. In the oil remaining in the pressure cooker, gently fry the onions and carrots for 2 minutes, then add celery, pepper, cloves, nutmeg, salt. Fry on the same heat for another 10 minutes. Add oxtail and heat for 5 minutes. Add soya sauce and turn heat to high for 2 minutes, stirring. Add water to cover, stir and put lid on pressure cooker. When steam starts to come out, reduce heat and start timing. After 35 minutes take off stove and cool down and remove lid. If you want to thicken the soup, mix the cornflour with hot water and stir into soup. Simmer for another 10 minutes without the lid, while stirring.
Serve with rice.