Preheat the oven to 300°F. Melt the tallow in a heavy, high sided pot over medium-high heat. While the oil heats, remove the potatoes from the water and dry very thoroughly with clean kitchen towels or paper towels. Line a baking sheet with paper towels and set nearby.
When the tallow reaches 370°F, carefully add some of the potatoes to the pot, taking care not to overcrowd. Fry the potatoes for 3 minutes, then transfer to the baking sheet with tongs or a spider. Fry the remaining potatoes in batches, letting the oil return to 370°F between batches.
When all of the potatoes have been fried, bring the tallow to 380°F. Working in batches, fry the potatoes a second time, until they are golden brown and crisp, about 3 to 4 minutes more. About 2 minutes before you remove the potatoes from the fat, toss the fresh sage into the oil and crisp along with the potatoes. Just before serving, season with sea salt. Eat hot.