Heat the butter in a non-stick frying pan. Reduce the heat, add the onions, sugar and salt, cover and simmer over a medium heat for approx. 25 mins., stirring occasionally until the onions turn a light brown colour, leave to cool.
Mix the sweetcorn with all the other ingredients up to and including the salt.
Mix the crème fraîche and coriander, season.
Place 1 tortilla in a hot frying pan without any oil. Spread ½ tbsp of crème fraîche on top, then add ⅛ of the cheese. Add ¼ of the onions and sweetcorn, sprinkle with ⅛ of the cheese, cover with a second tortilla, press down gently. Cover and fry for approx. 2 mins., turn and finish cooking for approx. 2 mins. until the cheese has melted. Repeat this step three more times. Serve with the dip and lime wedges.