potatoes (1-2 to person depending on size of potato)
salt, to taste
Peel the skin for a more refined french fry or scrub the skin well and leave it on for a rustic fry.
Cut potatoes into sticks between 1/4-inch and 1/2-inch wide. Rinse the cut potatoes in cold water to remove any excess starch.
To achieve better results, soak them in ice water for 30 minutes. Drain the potatoes before frying and pat them completely dry.
Melt duck fat in a heavy-bottomed pot with high sides. Heat the fat to about 325ºF, using a candy thermometer.
Cook the potato sticks in small batches to avoid dramatically dropping the temperature of the hot fat. After about 5 to 7 minutes, test the doneness by poking a fry with a knife. The knife should slide in and out with no resistance. If the potatoes are cooked through, remove them from the pot with a slotted spoon, spider skimmer, or tongs and let drain on a sheet tray covered in paper towels.
After all of the batches are cooked, raise the heat under the pot to bring the melted duck fat to 350ºF.
Return the fries to the pot, in batches again, for only about 1 minute. Drain on fresh paper towels.
Sprinkle them right away with your choice of sea salt. Doing this step while the fries are still hot will help the seasonings to stick.
The secret to the perfect French fry in any fat is the double-fry method. The first fry is to cook the potato through. The second fry at a higher temperature is to crisp them up.