Pour vegetable oil about 2 1/2 inches to 3-inches deep into 2 saucepans. Heat oil inone pan to 320 degrees F, and the other to 365 degrees F.
Peel the potatoes and trim the edges to form a rectangular football shape. Using a mandoline, make slices lengthwise 1/8 inch thick. Trim the end to round slightly. Do not wash the slices.
Drop slices into the 320 degrees F oil and gently shake back and forth on the heat for 6 to 7 minutes. Use a thick towel to prevent heat burn, and beware of splashing oil onto you. After about 5 to 6 minutes the slices should start to puff slightly and rise to the top. Keep shaking until most of your slices are puffed (about 3 minutes).
Remove the slices a few at a time and transfer into the hotter frying pan. They should puff right away. Move the slices around while frying so they cook evenly for about 2 to 3 minutes or until very crisp and golden brown. Remove the slices and drain on paper towels. Sprinkle some salt to season and serve immediately with a harissa aioli or Dijon mustard on the side.
Place egg yolks in a bowl. Add the mustard and whisk thoroughly. Add garlic and the harissa, and mix well. Slowly pour in olive oil while whisking rapidly (you can do this with a mixer or a bowl with a whisk attachment. Keep in mind, you would have to double the recipe to use the mixer). Refrigerate until needed.