1) Melt the butter in a large heavy-bottomed pot over medium heat. Add the onion and cook for 5 minutes. Add the garlic and cook for an additional 5 minutes.
2) Stir in the lemon zest, lemon juice, ketchup, tomato juice, brown sugar, molasses, Worcestershire sauce, chili powder, vinegar, chipotle puree, salt and 750ml water and bring to the boil. Lower the heat and simmer for 30 minutes, stirring frequently.