Fill a large pot with water and bring to a boil over high heat.

Drain the liquid from the jar of preserved grape leaves and remove them from the jar. Unroll the leaves and drop them into the boiling water. Reduce the heat to medium and let them cook for 5-8 minutes, then drain them in a colander and plunge them into a pan of cold water to cool them.

Gently separate the leaves and spread them on paper towels to drain. If the leaves have stems, clip them off with kitchen scissors. You will need about 50 or 60 leaves for the stuffing. 

Reserve any torn or small unusable leaves for lining the cooking pot.

Using a Dutch oven, braiser or heavy-bottomed pan, arrange the reserved torn grape leaves plus enough additional leaves to create a layer on the bottom of the pan. This is to keep the stuffed grape leaves from burning during the cooking process. Set the prepared pot aside while you make the filling. (See Notes)

In a large skillet, heat 3 tablespoons of the olive oil over moderate heat. Add the onions and cook, stirring frequently until they are soft and golden. Add the rice, salt and pepper and stir to coat with oil. Pour in the 3/4 cup of water, bring to a boil, reduce heat to low and simmer for about 5-8 minutes or until the water has been absorbed. Remove from the heat and allow to cool.

In a large bowl, add the ground lamb, egg, mint, dill, lemon juice, oregano, salt, and pepper. Add the cooled rice mixture and combine well.  

Spread the grape leaf flat, dull side up and place a portion (See Notes) of the filling on the center of the leaf. Turn up the stem end of the leaf, covering the filling.

Fold over each end to enclose the filling. Beginning again at the stem end, roll the grape leaf gently but firmly into a compact cylinder. The surface of the leaf will cling together to hold the grape leaf in shape.

Place the stuffed grape leaf, seam side down, in the prepared pot. Continue with the rolling process until all the leaves are filled or the filling is gone. Keep the stuffed grape leaves snug, side-by-side but not squeezed tight as they will expand as they cook. You can add another layer if needed.

Pour some broth or water into the pot. The water should almost cover the stuffed grape leaves, add more liquid if necessary. Drizzle the remaining 3 tablespoons of olive oil over the stuffed grape leaves and squeeze the juice from the 1/2 lemon over the stuffed grape leaves. Place an inverted dinner plate over the stuffed grape leaves. The plate will prevent the grape leaf bundles from unrolling as they cook. 

Set the pot over high heat and bring to a boil. Cover the pot with its lid and immediately reduce the heat to low and simmer for 30 minutes.  

Transfer the pot from the heat and remove the lid. Allow it to cool for 5-10 minutes. Transfer the dolmas to a plate, cover with foil to keep warm. Drain any remaining liquid from the pot and reserve 1 cup for the Avgolemono sauce.  

Into a food processor or bowl add the lemon juice and 1 tbsp of water, add the cornstarch and pulse or whisk well until dissolved.

Add the eggs and process or whisk well. Slowly add some of the warm broth and process or whisk quickly, add more broth and whisk again so that the eggs warm up. Then slowly pour and whisk the egg mixture into a small saucepan set over medium-low heat. Continue whisking and cook the sauce until it thickens slightly. Keep the heat adjusted to low and do not let the sauce boil.

Pour the sauce over the dish of lamb stuffed grape leaves and serve.