Rinse the grape leaves in running water and remove stems. Set aside.
To prepare the filling, in a heavy skillet over medium heat, sauté the onions in the olive oil for about 5 minutes, until softened. Stir in the rice, and cook for 5 minutes longer. Remove from the heat and add the tomato paste, parsley, dill, sugar, lemon juice, and season to taste with salt, pepper, and hot red pepper. Allow the mixture to cool before stuffing the grape leaves.
Preheat the oven to 350°F, and arrange the rack in the bottom third of the oven.
When the filling has cooled, lay the grape leaves with the underneath-side (the less smooth side with the visible veins) facing up, and place about 1 tablespoon of filling (or more depending on the size of the leaves) along the stem end of each leaf, fold over the sides, and roll from the stem up to form a sausage-like roll. Repeat with the remaining grape leaves. (If you have leftover filling, you can stuff bell peppers with it and cook it with the yalanchi.)
Arrange the yalanchi in a 9-inch square baking dish, lining them up 2 layers high. If you have extra unwrapped grape leaves, lay them across the top of the others. This will prevent the yalanchi from getting too dry or dark when baked. Pour water over the top to just cover the grape leaves.
Cover the pan with parchment paper, and then a layer of foil. Place in the oven and bake for about 1 ½ hours, or until the rice is puffed up and cooked through (taste one to be sure!). Remove the foil, but leave the parchment paper on it while it cools completely (this will prevent the top from drying out). After it cools to room temperature, cover and refrigerate until cold.
Serve cold, garnished with lemon slices. This dish is best prepared a day ahead, covered, and refrigerated until ready to serve.