Boil the potatoes: Place the potatoes in a large pot and cover with cold salted water by about 2 cm. Bring to a boil over high heat — you'll see vigorous bubbles breaking the surface. Reduce to a simmer and cook for 15-18 minutes, until a knife slides through the centre with almost no resistance. The skins should look slightly split and the potatoes will feel tender when pierced.

Preheat and prepare the tray: While the potatoes cook, preheat your oven to 220°C (200°C fan) or 425°F. Drizzle 2 tablespoons of olive oil onto a large baking tray and spread it evenly. Place the tray in the oven to heat up for 5 minutes — the oil should shimmer and release a faint nutty aroma when ready.

Smash the potatoes: Drain the potatoes in a colander and let them steam dry for 2 minutes — this removes excess moisture that would prevent crisping. Transfer them to the hot oiled tray in a single layer, leaving space between each. Using the bottom of a sturdy glass or a potato masher, gently press down on each potato until it flattens to about 1.5 cm thick. You'll hear a soft squish as the flesh spreads; the skin should crack open slightly, revealing the fluffy interior.

Season and roast: Drizzle the remaining 1 tablespoon of olive oil over the smashed potatoes. Sprinkle with the minced garlic, salt, and black pepper. Roast for 20-25 minutes, until the edges turn deep golden brown and crispy — you'll see the oil bubbling around the perimeters. The potatoes should feel firm and sound hollow when tapped with a spatula. Halfway through, flip each potato carefully with a thin spatula to ensure even crisping on both sides.

Make the dressing: While the potatoes roast, whisk together the mayonnaise, wholegrain mustard, and lemon juice in a small bowl. The mixture should be pale yellow, smooth, and slightly thick, with visible mustard seeds scattered throughout. Taste and adjust with a pinch more salt if needed.

Combine and finish: Remove the tray from the oven and let the potatoes cool for 5 minutes — they'll continue crisping as they cool. Transfer them to a large mixing bowl. Pour the dressing over the warm potatoes and toss gently using a rubber spatula, being careful not to break the crispy edges. Add the spring onions, chives, parsley, and capers if using, and toss once more. The salad should look glossy, with herbs clinging to the craggy surfaces and the dressing pooling slightly at the bottom.

Serve immediately or at room temperature: Transfer to a serving platter and sprinkle with the grated cheese if desired. The potatoes should be warm to the touch, with visible golden shards and a fragrant herbal aroma. Serve straight away for maximum crunch, or let it sit for up to an hour — the edges will soften slightly but remain satisfyingly textured.