Preheat the oven to 450F.
Cut a thin slice off the bottom of the potato so it lays flat. Place the potato in between two wooden chopsticks. Slice thin slices straight down, stopping at the chopsticks or before the chopsticks for the ends of the potato, since they may not reach the chopsticks. Aim to slice the slices about 2mm thick.
Place the potatoes on a parchment paper-lined baking sheet. Generously rub the potatoes with oil and season with salt and pepper, making sure to get in between the slices.
Bake for 45 minutes to 1 hour, until the potatoes are tender and the potato slices have begun to separate. Halfway through, brush with more oil, as needed.
While the potatoes are baking–in a bowl, mix together room temperature butter, scallions, cilantro, garlic, and Red Boat Fish Sauce.
Once the potatoes are tender, brush each potato with the fish sauce butter, getting in between the slices. Place it back in the oven and bake for another 25-35 minutes until it reaches your desired crispiness. Brush on some more fish sauce butter and enjoy immediately.