Preheat oven to 300ºF.

Arrange the cubed potatoes in a 9x13-inch baking dish. Pour the duck fat on top. Add more neutral oil as needed to ensure the potatoes are submerged. Nestle the garlic cloves into the potatoes and ensure you cover them with oil.

Add 2 teaspoons kosher salt. Scatter the sage leaves and rosemary sprigs on top.

Transfer to the middle rack of the oven. Roast, uncovered, for 1 hour 30 minutes. Check the potatoes occasionally.

Remove the baking dish from the oven and let stand for 10–15 minutes to cool slightly.

Carefully remove and discard the sage and rosemary sprigs.

Remove the garlic from the baking dish and transfer to a small bowl. Mash into a paste with a fork.

Use a slotted spoon to transfer the potatoes to a paper towel-lined plate. Cover with more paper towels on top and press the paper towels into the potatoes gently. Let stand for 10 minutes. After 10 minutes, press the paper towels into the potatoes again to blot off as much excess oil as possible. See note.

Heat 1 cup milk in the microwave until warm.

Place the potatoes and garlic in a large bowl. Use a potato masher to mash until mostly smooth. Add the milk in ¼ cup increments, and mash after each addition until the potatoes reach your desired consistency. I used 1¼ cup total, but you may use a bit more or less.

Add the minced sage, rosemary, and pinch of white pepper to the potatoes and stir to combine. Taste and add more salt and black pepper if needed. If desired, add a swirl of the duck fat from the baking dish for a more intense duck flavor.

Keep the potatoes warm until ready to serve. Before serving, garnish with a pinch of minced sage, rosemary, and a few cracks of black pepper. Enjoy!