Steps

1. Start the potatoes: place potatoes in a pot, cover with cold water, add a pinch of salt and bring to a boil. Cook 10–12 minutes until tender when pierced (small potatoes cook quickly).
2. While potatoes cook, make the lemon-Dijon yogurt: in a small bowl mix Greek yogurt, Dijon mustard, juice and a little zest from the lemon, a drizzle of olive oil, and a pinch of salt and pepper. Taste and adjust — it should be tangy and creamy.
3. Drain potatoes and halve or slice them thickly. Toss warm potato pieces with a drizzle of olive oil and a pinch of salt so they hold a light gloss.
4. Assemble stacks: on a plate lay a bed of arugula, arrange potato slices in a small stack or fan, spoon or brush some lemon-Dijon yogurt onto each potato piece, top with flaked mackerel, scatter capers and sliced spring onion over the top.
5. Finish: squeeze a little more lemon over everything, add cracked black pepper and an extra drizzle of olive oil if desired. Serve immediately while potatoes are warm for best contrast of textures.