Preheat the oven to 150C.
Start by prepping the pork. Roughly chop the pork into 1-inch chunks. In a large bowl mix together the fennel seeds, chilli flakes, ground cumin, orange juice and zest, tomato puree, chipotle paste, garlic cloves and salt to make a paste. Add the pork chunks to the paste and toss them so they are well coated in the spices.
In a large heavy bottom casserole pan, heat 1 tbsp olive oil. Add the pork and fry them off on all sides so they are golden brown, and the fat has rendered down – you might need to do this in batches. Once browned, remove with a slotted spoon. Keeping the pan on the heat, deglaze all the sticky bits that have stuck to the bottom of the pan, using 2 tbsp water and 2 tbsp honey. Let this bubble and scrape the bottom of the pan with a wooden spoon to lift it.
Then add the browned pork back into the casserole dish. Fill the pan with the vegetable stock, it should come up about halfway up the meat but if you need more then top up with water. Place the lid on the casserole and put it into the preheated oven for 2 hours.
While the pork is cooking prepare the pickled onions. Finely slice the onions and place into a small bowl. In a pan heat up the vinegar, sugar and Maldon salt. Heat it gently until the sugar and salt has dissolved, then pour the warm liquid over the sliced onions. Toss the onions and then allow them to sit. They will collapse and soften, turning a wonderful pink colour.
After 2 hours, check the pork. Remove the pan from the oven and using two forks, start to pull the meat apart – some parts will still be a little tough. Once you have pulled as much as you can, place the lid back onto the pan and put it back in the oven for a further hour.
Just before the meat is ready it is time to prepare the rest of the taco elements. In a small bowl mix together the natural yoghurt and chipotle paste. Destone the avocados and slice, then squeeze some lime over them to stop them from oxidising. In a small frying pan, heat up the tacos for a couple of minutes on both sides.
After 3 hours remove the pork from the oven. It should be meltingly soft by now and easy to pull apart with your forks.
Serve the tacos with a dollop of the chipotle yoghurt, some pulled pork, avocado slices, pickled red onions and some coriander leaves. Squeeze over some lime juice and a final pinch of Maldon salt, then enjoy!