Wash and optionally peel the potatoes, then cut into ¼-inch matchsticks and pat them dry.

Rinse the cut fries under cold water, then soak in ice water for 15 to 30 minutes.

Boil salted water with vinegar, add soaked fries, and boil for 3 minutes; then transfer to a wire rack.

Let fries cool completely on the wire rack for about 15-20 minutes.

Whisk together the flour, cornstarch, and Cajun seasoning; gradually mix in cold seltzer water.

Dip dry fries in the batter, ensuring they are evenly coated.

Heat avocado oil to 330°F and fry battered fries in batches for 4-5 minutes until lightly golden.

Chill fries in the freezer for about 15-30 minutes.

Increase oil temperature to 375°F and fry fries in batches for 3-4 minutes until golden brown.

Season with additional Cajun seasoning and salt immediately after frying, serve hot.