Place rendered duck fat into a large cast iron skillet, place in the oven to preheat.
Place potatoes in a large pot, cover with water. Add Kosher salt and baking soda, stir, then bring to a rapid boil. Cook until tender enough. Drain potatoes, immediately return to the pot.
Place lid on top, then secure by holding it firmly in place. Shake the pot vigorously, until the potatoes form a thin layer on top.
Cook undisturbed, then toss potatoes. Cook an additional 40 minutes, or until crispy and golden brown on all sides.
Garnish with finely chopped fresh herbs and sprinkle with flaky sea salt before serving immediately.