Set a saucepan with 1 tbsp oil over a medium heat. Cook the onion for 4-6 minutes until softened. Add the garlic and cook for 2 minutes more, then season and stir in the ketchup, honey, chilli flakes, Worcestershire sauce and vinegar. Briefly bring to the boil, reduce the heat and simmer for 8-10 minutes. Leave to cool, then store in
a bottle or airtight container.
When you’re ready to cook, drizzle the ribeye with the remaining oil and season generously. Brush all over with some of the barbecue sauce, place on the hottest part of the barbecue grill and cook for 6-8 minutes, turning every minute, until seared and coloured.
Move the meat to the coolest part of the barbecue. Cook for 45-60 minutes for medium-rare, or until done to your liking, basting with more barbecue sauce every 20 minutes.
Transfer to a plate and cover loosely with foil. Rest for 10 minutes before slicing. Serve any remaining sauce on the side.