1. Put the soaked pinto beans in a pan and cover with cold water. Bring to the boil over a high heat, then leave to bubble away for 10 minutes, skimming oﬀ any white froth on the surface. Turn the heat down slightly and simmer for another 10 minutes. Drain the beans and tip them into the slow cooker pot.
2. Grate three-quarters of the onion and save the remainder to serve. Add to the slow cooker pot with the oil, garlic, tomato purée/paste, cumin, smoked paprika, chipotle, thyme and hot water. Stir in the bouillon powder. Cover with the lid and cook for eight to nine hours on low (if you want to cook them overnight opt for the longer time) or five to six hours on high, until the beans are tender. Season with salt and pepper, to taste, then mash half the beans to break them down using a potato masher or the back of a fork.
3. To serve, ﬁnely dice the remaining red onion and put it in a bowl with the tomatoes and coriander/cilantro. Season with salt and add a good squeeze of lime juice. Place the avocado on a separate serving plate and squeeze over some lime juice to stop it turning brown. Crumble the feta into a bowl.
4. Let everyone help themselves, spooning a good helping of the beans onto a warm tortilla and topping it with a few slices of avocado, the tomato, salsa and a sprinkling of feta, if using. Finish with a drizzle of olive oil and a squeeze of lime juice.