Peel the potatoes and cut them into rough two-inch chunks. It’s fine for them to be slightly uneven, as this creates different textures as they cook.
4 large russet potato (~3 pound / 1.36 kilogram)
Place the potatoes in a large saucepan with the cold water, kosher salt, and baking soda. Starting with cold water helps the potatoes cook evenly without the outsides turning to mush before the centers are cooked.
2 quart cold water, 3 tablespoon kosher salt, 1 teaspoon baking soda
Place the saucepan over medium heat and bring the water to a gentle simmer.
Cook for about ten minutes, or until the potatoes are very tender but not falling apart. Take them as far as you can without letting them break apart, as this is key to achieving fluffy, crispy roast potatoes.
Drain the potatoes carefully, then arrange them on a wire rack. Transfer to the refrigerator, ideally overnight, to allow them to steam-dry.
Preheat the oven to 425°F (220°C). Add two-thirds of the neutral oil and the beef tallow or duck fat to a deep baking tray and place it in the oven to heat. For a vegetarian option, replace the animal fat with additional neutral oil.
⅓ cup neutral oil, ⅓ cup beef tallow or duck fat
Meanwhile, transfer the potatoes to a large bowl and dust them with the potato starch. Drizzle with the remaining neutral oil and toss carefully until evenly coated.
2 tablespoon potato starch
Carefully remove the hot baking tray from the oven and add the potatoes to the hot fat.
Return the tray to the oven and roast the potatoes, without turning, for 25 minutes to allow the crust to set.
Remove the tray from the oven and turn the potatoes over. Return them to the oven and roast for another 25 minutes.
Remove the tray and carefully drain off any excess fat.
Return the potatoes to the oven for a final ten minutes, or until crisp and golden brown.
Toss the potatoes with the thyme, crushed garlic, and a pinch of flaky salt. Serve immediately with your favorite dipping sauce.
2 clove garlic, 2 teaspoon fresh thyme, flaky salt