Cut potatoes and immediately put into a large bowl of cold water. Cut potatoes will turn an ugly grey if left out in the air. Allow to soak for 30 minutes up to a few hours then drain and dry on paper towels. Heat a large, deep pot of oil to 300 and fry small batches of fries for 4 minutes, keeping the oil at 300. They won't brown, just get partly cooked.
Drain and cool on paper towels, then move to a parchment lined baking sheet and arrange fries so they mostly aren't touching much so you can fry small batches later. Freeze for 1 hour or overnight.
Heat oil to 375. Fry small batches of frozen fries for 2-3 minutes until golden. Drain on paper towels and sprinkle with salt. Serve immediately.