Cook the new potatoes in a saucepan of boiling water, until tender.
Drain and toss the potatoes with the olive oil, spring onions and beetroot in a large bowl.
Place the mackerel fillets, skin side down, on a baking sheet and squeeze over the lemon juice and season with the pepper. Cook under a preheated hot grill for 3–4 minutes on each side, until cooked through.
Meanwhile, toss the watercress into the potato salad, and stir the capers and lemon rind into the yogurt in a small jug.
Divide the salad between 4 plates and top each with 1 grilled mackerel fillet and a spoonful of yogurt dressing. Serve with lemon wedges.