Put the potatoes in a wide-based saucepan, add chicken stock, lemon zest and garlic (the stock may not cover the potatoes fully, that’s OK). Bring to the boil, cover and cook for 8 minutes.
Drain potatoes, picking out the lemon zest and saving it for later.
Oil an oven tray generously. Gently flatten each potato. I found the best method was to flatten them on a chopping board using the base of a small saucepan, but you could use the side of a chef’s knife or a rolling pin. Flattening them on the oven tray is possible, but the oil makes the surface slippery and the potatoes tend to dart around the tray when you try to flatten them.
With the smashed potatoes on the oven tray, drizzle them with olive oil and lay the reserved lemon zest in between the potatoes. Roast for 40 minutes, until the potatoes are crisp, golden and irresistible.
Sprinkle with sea salt and finely grate parmesan over the top.