To make the sauce, heat the oil in a pan and sauté the onions until soft (about 10 minutes), add the garlic and cook for a three minutes or so. Add the chipotles, tomatoes and sugar, season, and bring to the boil. Turn the heat down low and cook uncovered for 20 to 30 minutes, stirring every so often. You should have a thick sauce, but make sure it doesn’t taste too intense – add a little water if you’ve cooked it too far.