Preheat the oven to 140°C fan/160°C/gas 2.
Tip the onion, garlic, cumin, oregano, cloves, citrus juice, achiote paste and salt into a blender or food processor and blitz until smooth.
In a small deep roasting tin or lidded casserole dish, mix the pork shoulder with the spice paste. Cover tightly with foil or the lid, then transfer to the oven and cook for 3 hours.
Meanwhile, mix the very thinly sliced red onion with the lime juice and set aside for 3 hours, stirring occasionally. (The acid in the lime juice will turn the onions a beautiful bright pink by the time the pork is ready.)
Once cooked, remove the foil or lid and shred the pork while hot. Serve with the pink pickled onions, chopped coriander, warm tortillas and sour cream.
