Place the potatoes in a large saucepan and cover with cold salted water. Bring to the boil and cook for 15–20 minutes, until very tender when pierced with a knife.
Drain well and leave them in the colander for 5 minutes to steam dry. Removing excess moisture helps them crisp in the oven.
Preheat the oven to 220°C (200°C fan) and line a large baking tray with baking paper.
Spread the potatoes out on the tray with space between each one. Using a potato masher or the base of a glass, gently press down until each potato is about 1.5–2 cm thick with rough, craggy edges.
Drizzle evenly with olive oil, then sprinkle with garlic powder, thyme, salt and pepper.
Roast for 35–45 minutes until deeply golden and crisp around the edges and underneath. Flip halfway through if you’d like extra crunch on both sides.
Finish with flaky sea salt and a little lemon zest or parsley. Serve hot.