Mash garlic and salt with the side of a chef's knife. (Alternatively, mash garlic and salt in a mortar with a pestle.) You should have about 1 tablespoon mashed garlic. Set aside.Advertisement
Whisk together yogurt, egg and cornstarch in a heavy medium saucepan. Set over medium-low heat and cook, whisking constantly, until mixture thickens; do not let it boil. Transfer to a small bowl. Whisk in reserved garlic, oil and lemon juice. Taste and adjust seasonings. Press a piece of plastic wrap directly on the surface to prevent a skin from forming. Let cool.
Make Ahead Tip: Cover and refrigerate for up to 3 days; whisk briefly before using.