Place the potatoes in a large pot of cold, well-salted water. Bring to a boil and cook 8–10 minutes, until just tender on the outside but not fully cooked. Drain completely and let steam-dry for 5 minutes.
Shake the potatoes gently in the pot to rough up the edges. This creates maximum surface area for crisping.
Preheat the oven to 425°F (220°C). Add the duck fat to a heavy roasting pan or cast-iron skillet and heat it in the oven for 5–7 minutes, until smoking hot.
Carefully add the potatoes to the hot duck fat and toss to coat. Season with salt, pepper and herbs if using.
Roast 45–55 minutes, turning every 15–20 minutes, until deeply golden and shatter-crisp on all sides. Add the garlic for the final 15 minutes so it doesn’t burn.
Transfer to a serving bowl. Taste and finish with flaky sea salt and fresh herbs if desired.
