Preheat the oven to 190C/375F/Gas 5.
Parboil the potatoes in salted water for 7-8 minutes until almost cooked (they will still feel firm when pierced with a knife). Drain thoroughly using a colander, then shake the potatoes around a little in the colander until the outsides are roughened.
Put the duck fat for the roast potatoes into a small roasting tin and heat in the oven for five minutes.
Add the drained, roughened potatoes to the tin of hot duck fat, then sprinkle over the garlic cloves, thyme and sea salt and mix until the potatoes are well coated in the fat.
Return the tin to the oven and roast for 45-60 minutes, or until golden and crunchy.