Add the potatoes to a pot, add enough cold water that they are barely covered, and bring them to a boil over medium heat. Cook until just done.
Drain the water and peel the potatoes as soon as you can comfortably touch them.
Let the potatoes cool down completely.
Cut the potatoes into even slices.
In a cast-iron skillet, heat the fat over medium-high heat until very hot.
Carefully add the potatoes to the skillet in a single layer (you might have to do this in batches) and fry them until golden brown and crisp.
Flip the potato slices and fry them on the other side.
Remove the fried potatoes with a slotted spoon or spatula. If you like you can place them on a paper towel to soak up any excess fat.
Continue with the remaining potato slices. You might have to add more fat as needed.
Season the potatoes with sea salt and plenty of black pepper.