Prep the Potatoes – Wash, peel (if desired), and cut potatoes into ¼-inch thick strips.
Rinse & Soak – Rinse in cold water, then soak in ice water for at least 15 minutes.
Parboil the Fries – Bring a large pot of water to a boil. Add salt and vinegar, then boil fries for 4-5 minutes. Drain and let dry completely.
Bake the Fries – Toss fries with avocado oil, spread them in a single layer on a parchment-lined baking sheet, and bake at 425°F for 40-45 minutes, flipping halfway through.
Make the Cheese Sauce – Melt butter in a saucepan over medium heat. Whisk in flour and cook for 2-3 minutes. Slowly add milk, whisking constantly, until thickened. Lower heat, add cheese, and stir until melted. Add pickled jalapeño purée, pickle juice (or vinegar), salt, chili powder, and paprika. Keep warm.
Season the Fries – As soon as the fries are done, sprinkle with nacho fries seasoning while hot.
Serve – Pile fries onto a plate and serve with warm nacho cheese on the side or drizzled on top.