Omelette des Oignons et des Frites

Place potatoes in a medium bowl and pour in water to cover; let soak until ready to cook.

Pour water into a large Dutch oven or other heavy pot to fill halfway full; season generously with salt. Bring to a simmer over medium heat. Drain potatoes and add to pot; cook until fork-tender, 6–8 minutes (be careful not to overcook). Using a slotted spoon or tongs, transfer potatoes to a wire rack set inside a rimmed baking sheet and let cool and dry. Transfer potatoes to a rimmed baking sheet lined with paper towels and pat dry (for the crispiest results, you want to make sure your potatoes are completely dry before frying).

Pour out water and dry pot. Pour in oil to come 1½" up the sides and clip thermometer onto pot. Heat oil over medium-high until thermometer registers 375°. Working in 2 batches, fry potatoes until light golden brown, 11–14 minutes per batch. Using slotted spoon or tongs, transfer pommes frites back to rack as you go; immediately season with salt (potatoes will darken as they cool).