Add the onion, lime juice, olive oil, cumin and chilli powder to a bowl, and mix well.
Mix in the plums, tomatoes and coriander. Cover and set aside for 5 minutes.
Meanwhile, make three slits across the skin of each mackerel fillet.
Grill or barbecue each mackerel fillet skin side up under a grill, or skin side down on a barbecue, for around 4 minutes. This should be enough to cook the fillet right through but if not, turn it over and cook it for 1 minute on the flesh side too.
After warming the tortilla, add some salad leaves and a mackerel fillet. Top it with a dessertspoon of salsa and roll up.
Serve the rest of the salsa on the side.