Heat the oil in a large non-stick flameproof casserole dish or saucepan. Fry the onions with a pinch of salt for 10 mins, or until softened. Add the garlic and fry for 1 min more. Tip in the chipotle paste, tomatoes and sugar with half a can of water. Sit the chicken thighs in the sauce. Bring to a low simmer and cook, covered, for 20-25 mins, stirring often, until the liquid has reduced slightly and the chicken is cooked through.
Using two forks, shred the chicken and stir it back through the sauce. Season to taste. If the sauce is too thin, return to the heat and simmer for 5 mins until the sauce has reduced more. To freeze, leave to cool completely and transfer to large freezerproof bags.
To make the salsa, combine the red onion, tomatoes, lime zest and juice, coriander leaves and stalks. Season to taste. Serve the chicken with the tomato salsa, tortilla chips, guacamole and a scattering of coriander leaves.