1. Put the pork in a large non-metallic container. Toast the fennel seeds, cumin seeds and peppercorns in a dry frying pan over a medium heat for 3-5 minutes, or until fragrant and starting to colour slightly. Grind in a pestle and mortar or mini processor until fine, then mix with the paprika, lemon juice, orange juice and vinegar. Pour over the pork, add the cinnamon stick, cover and leave in the fridge to marinate for at least 4 hours, or preferably overnight.
2. Preheat the oven to 170°C, gas mark 3. Transfer the pork and marinade to a large roasting tin. Add the stock and cover tightly with foil. Bring to the boil over a high heat then transfer to the oven and cook for 3-4 hours, or until tender and falling apart, removing the foil for the last hour.
3. To make the sauce, cover the chillies with boiling water and leave to soften for 15 minutes, then remove and split in half. Discard the seeds and stalks, then whizz the chillies to a paste in a mini processor with 2-3 tbsp of the soaking water. Place in a saucepan with the rest of the sauce ingredients, mix well and bring to a boil over a high heat. Simmer for 10 minutes, until thickened slightly.
4. To serve, mix the sliced red onions with the juice of 2 limes. Season and set aside for at least 10 minutes, until softened slightly. Meanwhile, using a teaspoon, scoop the avocado flesh into a bowl and squeeze over the juice of the remaining lime. Season to taste. Shred the pork with two forks, discarding any fat and skin, and place onto a warmed serving platter. Spoon the sauce into a bowl and serve with the pork, avocados, coriander and pickled red onions. When ready to eat, top a tortilla with the pork, sauce and accompaniments, then fold or roll up and enjoy.