To make the marinade, blitz the garlic, chilli and ginger in a blender until finely chopped. Add the soy or teriyaki sauce and ketchup, then pulse until completely blended. Put the beef in a dish and smear over the marinade. Leave in the fridge to marinate for up to 24 hours, the longer the better.
To cook the beef, fire up the barbecue and, when flames subside and coals glow, lay it fat side down and char for about 5 mins to get fat sizzling. Flip it over and cook for 40 mins for medium rare, turning occasionally until all sides are charred. If the barbecue has a lid, use it for a smokier flavour. A few mins before taking the beef off, drizzle over brandy or bourbon (if using) to flambé the meat.
Lift the meat onto a board, cover with foil and leave to rest for at least 10 mins. Carve the beef into slices like a roast and serve with any juices.