Preheat the oven to gas 6, 200°C, fan 180°C. Spoon the goose fat into a large shallow roasting tin and place in the oven for 5-10 minutes to heat.
Meanwhile, scrape the rounded side of each potato with a fork to make deep grooves in the surface. Remove the roasting tin and carefully add the potatoes to the hot fat. Scatter with garlic cloves, half of the rosemary and some sea salt, then roast for 45 minutes, turning occasionally, until golden and crunchy.
Remove the potatoes and drain on kitchen paper. Scatter with a little more sea salt and rosemary to serve.
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