Drain the soaked beans and rinse under cold running water. Put them in a large saucepan, cover with plenty of fresh water and add the quartered onion, 2 peeled garlic cloves and the bay leaves. Bring to the boil and boil rapidly for 10 minutes, then reduce the heat, partly cover and simmer gently for 45–60 minutes or until tender.
Meanwhile, make the salsa by mixing together the tomatoes, chilli, lime zest and juice, sugar and coriander in a bowl. Cover and set aside until ready to serve.
When the beans are cooked, spoon out 150ml of the cooking liquid and reserve. Drain the beans, discarding the onion and bay leaves but reserving the garlic.
Heat the oil in a large frying pan, add the finely chopped onion and garlic, then cook gently for about 10 minutes or until soft. Add the reserved whole garlic cloves, a ladleful of the beans and a few spoonfuls of the reserved cooking liquid. Mash with a fork to break up the beans and garlic cloves. Continue adding the beans, a ladleful at a time, together with a little of the liquid, mashing over a low heat to make a dryish, slightly textured purée. Season with pepper.
Meanwhile, heat the tortillas in the oven or in a microwave according to the packet instructions.
Spoon the refried beans into the middle of the tortillas. Sprinkle with the cheese and the shredded lettuce, then add the yogurt. Roll up the tortillas to enclose the filling and serve immediately, with the tomato and chilli salsa.