Heat the oil in a medium saucepan. Add the onion and cook for 5 mins or until softened and starting to turn golden, adding the garlic for the final min. Stir in the sugar, chipotle paste and tomatoes. Put the chicken into the pan, spoon over the sauce, and simmer gently for 20 mins until the chicken has cooked (add a splash of water if the sauce gets too dry).
Remove the chicken from the pan and shred with 2 forks, then stir back into the sauce. Scatter with a little red onion, the coriander, and serve with remaining red onion, tortillas or rice.