In a bowl, combine all ingredients for the Za’atar. Store in an airtight container.
To a Vitamix blender, add all ingredients for the Herbed Yogurt, seasoning with salt and lemon; purée. Adjust seasoning, as necessary. Cover and chill.
In a bowl, toss to combine onions and sumac for the Sumac Onions; reserve.
Soak wooden skewers for 24 hours. Heat water bath of an immersion circulator to 74°C.
In a large bowl, season lamb with salt, oil, and 200 grams Za’atar. Seal in vacuum bags.
Cook lamb sous vide for 12 hours. Remove from water bath and cool.
Cut lamb into 30-gram cubes. Prepare and heat grill.
Place 3 lamb cubes on each wooden skewer. Sear on all sides.
Garnish each kebab with 30 grams Herbed Yogurt and 30 grams Sumac Onions.
