Wash and peel (if you want) 3 large russet potatoes. Cut them into ¼-inch matchsticks. Rinse in cold water 2–3 times and soak in ice water for 15 minutes.
Bring a pot of salted water with 1 tablespoon of vinegar to a boil. Boil the fries for 3 minutes. Remove them with a slotted spoon and place them on a wire rack to cool and dry completely.
In a bowl, mix flour, cornstarch, Cajun seasoning, and cold seltzer or soda water until smooth. Dip the dried fries into the batter, shaking off any extra.
Heat 4–5 cups of avocado oil in a pot to 330°F. Fry the battered fries in small batches for 4–5 minutes. Place them back on the wire rack.
Freeze the fries for 15–30 minutes to help them stay crispy later.
Heat the oil to 375°F. Fry the chilled fries in small batches for 3–4 minutes, or until they’re golden and crispy. Drain on paper towels.
While the fries are still hot, sprinkle with more Cajun seasoning and a pinch of salt. Serve warm with your favorite dipping sauces.