Squeeze the juice of the lemon and put it in a bowl. Quarter the apple and remove the core. Cut the apple into thin slices and toss with the lemon juice. Preheat the oven to 180°C/fan 160°C/Gas 4.
Remove the outer leaves from both of the cabbages and then cut out and discard the tough pale inner core. Shred the cabbages as finely as possible and put in the bowl with the apple. Peel and coarsely grate the carrots and finely slice the spring onions then add to the same bowl. Toss the apple and vegetables.
In a small bowl, mix the mayonnaise with the Dijon mustard and water until smooth. Stir in the yogurt and pour over the salad. Mix well together and set aside. Place the mackerel fillets on a baking tray, skin side up, and heat through in the oven for 6-8 minutes until hot. Divide the coleslaw between six plates, add a warm mackerel fillet to each one and garnish with fresh watercress.
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