Brush the Porterhouse steak on all sides with rapeseed oil and season with salt and a pinch of cane sugar. Heat the grill to high (280-300°C / 536-572°F). Sear the steak on each side by grilling for 10 seconds, turning 90° halfway through each side, for a total of 20 seconds per side.
In a bowl, combine the cola (approx. 150ml), chopped coriander, finely chopped red chilli, lemon zest from one small lemon, 4 tablespoons rapeseed oil, salt, and pepper.
Place the steak in a roasting basket with a catching tray. Set the grill to indirect heat at 100°C (212°F). Grill for 90 minutes, basting the steak with the marinade at 30 minutes, 60 minutes, and 75 minutes. The steak is done when the internal temperature reaches 55-59°C (131-138°F) for medium rare.
Remove the steak from the grill and rest for 5 minutes. Use a narrow knife to remove the meat from the bone. Slice and serve.