Begin by peeling the yellow potatoes and cutting them into large chunks; leave smaller potatoes whole, halve medium ones, and quarter the largest. Rinse the cut potatoes under cold water to remove excess starch, then set them aside.

Bring a large pot of salted water to a rolling boil, then gently add the prepared potatoes. Parboil them for about 10 minutes until just starting to soften but still firm in the center. Drain and transfer to cold water.

Preheat your oven to 425°F (220°C) and pour duck fat into a roasting pan, coating the bottom evenly. Place the pan in the oven to melt the fat.

In a mixing bowl, combine the parboiled potatoes with baking soda, kosher salt, cracked black pepper, chopped rosemary, and whole garlic head. Toss to coat evenly.

Carefully remove the hot roasting pan from the oven and add the seasoned potatoes, spreading them in a single layer. Roast for about 1 hour, turning every 20 minutes.

In the last 5 minutes, drizzle olive oil over the potatoes and sprinkle with flaky sea salt before serving.