Place the annatto seeds in a small saucepan, cover with a small cupful of cold water, bring to the boil, then simmer over a low heat for 20 minutes. Remove from the heat and allow the seeds to steep for a few hours, then drain them in a colander, reserving the liquid.
Preheat the oven to 200°C/400°F/gas 6. Peel the onion, cut into 8 wedges and place in a small roasting tin with the garlic. Roast for 45 minutes or until soft. Transfer to a plate, allow to cool, then remove the garlic skins. Roughly chop the garlic and onion on a wooden board.
Place the garlic and onion mixture in a food processor with the drained annatto, allspice, coriander seeds, cinnamon, cumin seeds, oregano, saffron, chilli powder and 1 tablespoon of sea salt, and blend to a rough paste.
Squeeze in the lime juice and add the orange juice, cider vinegar and about 4 tablespoons of the reserved annatto liquid, then blend briefly again to combine. Transfer the mixture to a large nonreactive bowl.
Place the pork chops in the marinade, cover with clingfilm and marinate for 3 to 4 hours or overnight in the refrigerator.
Remove the pork chops from the fridge and bring to room temperature. Preheat a barbecue or cast-iron griddle pan to a medium heat. Cook the pork chops, turning once, for 6 to 10 minutes on each side depending on thickness, occasionally brushing on some more marinade during cooking. When done, move the pork chops to a board and leave them to rest for a couple of minutes.
For the onion rings, place the flour and chilli powder in a large bowl, season well and mix to combine.
Thinly slice the onions on a mandolin (use the guard). Very carefully heat the oil in a large saucepan to 180°C (always be alert when deep-frying). Place the onion slices, in batches, in the flour and coat well. Shake off the excess flour and carefully lower the onion slices into the oil. Cook for 1 to 2 minutes, in batches, until golden brown. Remove with a slotted spoon, then drain on kitchen paper and keep them warm while you cook the remaining onions.
Transfer the onion rings to a bowl and serve with the pork chops, whole or cut up.