Fillet the mackerel and cut each fillet into four pieces.
Peel the onions, scald and skin the tomatoes and halve the mushrooms.
Remove the stalk and seeds from the pepper and cut it into 1.6cm (1/2 in) strips.
Thread a tomato onto each of four kebab skewers and then alternate the pieces of mackerel with the onions, mushrooms and pepper. End with another whole tomato.
Put the kebabs onto a large, shallow dish, big enough to take them all lying flat.
Beat the remaining ingredients together and pour them over the kebabs.
Leave the kebabs for at least 2 hours in a cool place but preferably not the refrigerator, turning them several times.
When you are ready to cook, preheat the grill (broiler) to high.
Lay the kebabs on the hot rack and baste them with any marinade remaining in the dish.
Cook them for 12 minutes, turning them frequently.
Serve the kebabs with a savoury brown rice.
