Chu-Argentinean Sandwiches-Photo1

Whisk the egg yolk, lemon juice, mustard, salt, and 1 teaspoon of water in a medium bowl until combined and lightly frothy. Slowly drizzle in the oil while continuing to whisk the egg mixture. Continue adding the oil just until the mixture begins to emulsify and thicken.Keep refrigerated in a tightly sealed container for no longer than 1 week.