Combine the chillies and garlic in a mortar with the fennel, cumin, nutmeg, pepper, and salt. Grind the mixture to combine and break up the whole seeds. Rub this mixture all over the surface of the pork.
Slice the onions and set in the bottom of a deep roasting tray, then pour over the RC Cola and vinegar. Place the rubbed shoulder on top of the onions and cover with foil.
Roast for 4 hours. Remove the pork from the roasting pan and allow to cool slightly.
Strain the liquid from the pan into a saucepan and set over a low heat. Reserve the onions and any other solid bits.
Add the tomato ketchup and Worcestershire sauce to the drippings and reduce to a thick syrupy consistency.
To pull the pork, use two forks as claws, shredding the meat by pulling in opposite directions. Combine the meat with the reduced sauce and strained onions.
Serve on baps with a little mustard, a few leaves of lettuce, thinly sliced onion and some chilli sauce.