Pour canola oil into a deep cast-iron skillet to a depth of 2 inches. Heat over high heat to 350 degrees F. Line a plate with paper towels.
While the oil is heating, lightly scrub the potatoes with a firm brush under running water. Use the fine julienne blade on a mandoline to cut the potatoes. (Alternatively, very thinly slice the potatoes lengthwise. Stack the slices and very thinly slice them lengthwise to make long thin straws.) Rinse the potato straws under cold running water. This rinses off the starch and makes the straws nice and crispy. Pat dry on paper towels.
Working in batches, fry the potatoes in the oil until golden brown and crispy, 2 to 3 minutes. With a slotted spoon or spider, transfer the straws to the paper towel¿lined plate. Season with salt and pepper immediately and serve.