Preheat oven to 190C/Gas 5/fan oven 170C. Heat the oil in a large frying pan, add the onions and cumin seeds and fry until softened.
Tip the beans into the pan with the onions and start to warm. Halve the tomatoes, chop the coriander and dice the feta. Add to the pan and gently heat just until bubbling.
Place the tortillas directly on the oven shelves and heat for 1-2 minutes until warm and starting to puff. Now get everyone to help themselves and create their own tacos. Pile pieces of lettuce on the warmed tortillas and add a spoonful of the spicy bean and feta mixture. Top with a spoonful of guacamole, and an olive if you like. Fold over and serve.