Quickly pickle the onion: put it in a bowl and sprinkle with the sugar and 1⁄2 tsp salt. Leave for 10 mins, then stir in the cider vinegar and leave to pickle for another 10 mins.
Meanwhile, stir the beans and chilli sauce into the Bolognese and heat for 10-15 mins until piping hot. Add a little water if the mixture seems too thick.
Heat the corn tortillas following pack instructions. Top with spoonfuls of the Bolognese, some pickled onions, lettuce, avocado and a dollop of soured cream.