Preheat oven to 200°C / 400°F (fan)
Peel and cut potatoes into even large chunks with a sharp knife
Parboil in cold salted water for 8 to 10 minutes until just tender (not falling apart)
Drain, shake gently to fluff edges, and let steam dry for 10 minutes
Heat duck fat in a heavy roasting tray until shimmering hot
Add potatoes, garlic, and thyme. Season lightly
Roast for about 60 minutes, turning occasionally, until deeply golden and crisp
Discard garlic, finish with chopped thyme and extra salt
