The finished recipe. Grilled Mackerel with Mustard Potato SaladIngredients 1 to 6. 140 g Mackerel fillet (Weighed raw, skin on unless the method says otherwise. Cooked weight is roughly 20% less for the same fillet.). 180 g New potatoes (Raw weight, skins on.). 2 tsp (12 g) Wholegrain mustard. 50 g Greek yogurt, 0% fat. 20 g Spring onion. 30 g GherkinsIngredients 7 to 8. 30 g Watercress. Of half a lemon Lemon juiceStep 1. Boil the potatoes for fifteen minutes, drain, and halve them while hot.Step 2. Stir the mustard, yogurt, half the lemon and a pinch of salt together, and toss the warm potatoes in it with the chopped gherkins and spring onion.Step 3. Score the mackerel skin three times, season, and grill it skin-side up under a hot grill for six minutes without turning. The skin should blister and the flesh flake.Step 4. Serve on the watercress with the potato salad and the rest of the lemon.