Heat a small skillet over medium high heat, and sauté the chopped coriander and crushed garlic with a pinch of salt until the garlic has just turned golden. Set aside.
Mix the cornstarch with just a splash of cold water to create a paste.
Using a blender or immersion blender, blend together the yogurt, the egg, the cornstarch paste, and the salt until well combined.
Pour yogurt mixture into a heavy saucepan over medium heat and immediately start whisking. Keep whisking and don’t stop or the sauce will separate. After 8-10 minutes or so, the sauce will begin to boil and as soon as that happens take it off the stove.
Add the fried garlic and coriander mix, stir and taste for seasoning.