In a large saucepan of boiling water scald 36 grape leaves in batches. Use a slotted spoon to remove leaves, then plunge them into a bowl of ice-water. When cool enough to handle, cut off stems. Pat leaves dry and place shiny-side down on paper towels while you prepare filling.
In a skillet, heat onion and oil over medium heat, stirring, until tender. Spoon onion and oil into a small bowl. In same skillet cook lamb, breaking apart clumps with a fork, until lamb is no longer pink. Pour lamb into a sieve to drain fat. In skillet combine lamb, onion and rice. Stir to coat rice with juices. Add tomatoes and 1/4 cup of the stock and cook 10 minutes, or until rice has absorbed liquids. Stir in allspice, herbs and season to taste with salt and pepper. Cover and let sit 10 minutes.
In center of grape leaf place 1 teaspoon of lamb mixture and fold sides over; roll up into a tight cylinder. Repeat with remaining leaves. In bottom of a large casserole, arrange 2 layers of stuffed leaves and sprinkle with lemon juice. Pour remaining broth over stuffed leaves to cover. (Add more water if necessary.) Top with a weighted heat-proof plate to submerge stuffed leaves. Bring casserole to a simmer over medium heat, reduce heat to low, cover and simmer 35 minutes, or until rice is tender. Serve stuffed grape leaves hot with a spoonful of Yogurt sauce.
Yogurt Sauce: In a cheesecloth-lined sieve set over a bowl let 1 1/2 cups yogurt drain overnight, refrigerated. Chop 2 garlic cloves with salt to taste until it forms a paste. In a small bowl, combine yogurt and garlic paste, stirring well to mix. Yield: About 1 cup.;