Carefully separate the grape leaves, place in a large bowl and pour boiling water over them to cover. Let the leaves soak for 20 minutes, then drain and rinse to remove excess salt. Drain the leaves, snip off the stems (reserving stems), and lay the leaves on a towel to dry.

In a saucepan, bring 2 cups of salted water to a boil, and stir in the rice. Cover, reduce the heat to low and cook rice until water is absorbed, about 17 to 20 minutes. Meanwhile, heat 2 tablespoons olive oil in a skillet, add the onion and saute until soft, about 3 to 5 minutes. Add the garlic and saute one more minute, until aromas are released. Add the lamb and cook until the meat is well browned, breaking it apart with a fork while cooking, about 15 minutes. Add the oregano, and salt and pepper to taste. Stir in the feta and remove from the heat. Stir in the rice, parsley and mint.

Place one leaf on a flat surface, vein side up, shiny side down. Place a rounded teaspoon of filling in the center of the leaf, near the stem edge. Fold the stem end over the filling, then fold both sides toward the middle, and form into a roll. Squeeze lightly in the palm of your hand to secure the roll. Repeat process with remaining leaves and filling.

Line the bottom of a 3quart heavy saucepan with reserved stems, trimmings and any leftover or torn grape leaves, and arrange bundles seam sides down, packing them close together in layers.

Combine the remaining 1/4 cup olive oil with 3/4 cup water, the sugar, and lemon juice, and pour over the stuffed grape leaves. Place a small, heatproof plate on top of the stuffed leaves, cover the pan and simmer over low heat for about 45 minutes to 1 hour, or until leaves are tender and most of the liquid is absorbed. Serve warm, garnished with lemon slices and mint leaves.