First, make the relish. Cut the stalks off the aubergines, then cut the flesh into 1cm cubes. Heat 1½ tbsp of the oil in a large frying pan and fry the aubergine in batches over a medium heat for 8-10 mins, or until softened and golden. You may need to add more oil. Transfer to a plate and set aside.
Add another ½ tbsp oil to the pan and fry the onion until softened and golden, about 10 mins. Tip in the tomato and cook for another 10 mins, or until collapsed, then add the garlic, chilli, cumin and ginger, and cook for 2 mins more. Return the aubergine to the pan, add the honey, the juice from the jar of preserved lemons and the dates. Season. Turn the heat down and simmer for about 10 mins, or until thick but not stiff – add a splash of water if you need to. Chop the preserved lemon – pulp as well as skin – discarding any seeds, then add half to the relish. Taste to see if you need more – it should be assertive, sweet and savoury. You may want more honey as well. Leave to cool, then stir in the mint.
Toast the cumin and coriander seeds in a dry frying pan over a low heat until the seeds start to crackle and jump in the pan. Transfer to a pestle and mortar and crush to a coarse mixture. Add salt and pepper.
Rub olive oil all over the mackerel fillets, then sprinkle them with the spice mix. Heat a frying pan or griddle pan and, when it’s very hot, add the fish fillets, skin-side down. Cook for 2-3 mins. Turn them and cook for 2 mins more. Squeeze lemon juice all over the fillets and serve them with the relish. Serve with bulgur wheat or couscous and coriander, if you like.