Lightly flour the pieces of pork belly. Heat some sesame oil in a thick-bottomed saucepan or flameproof casserole dish and quickly colour the pieces of pork on all sides.
Add the onions, chillies, ginger, star anise, garlic, five-spice, soy and fennel seeds. Pour over the meat stock, bring it to the boil, cover and simmer gently for 2½-3 hours until the pork is very tender. If the liqui