To cook:Add half of the cooked beans in one layer to the heavy based pot with the meat in it. Arrange the stuffed leaves side-by-side, seam side down, over the beans packing them tightly in concentric circles. Place the sliced tomatoes over the stuffed grape leaves. Scatter over the remaining beans. Weigh the stuffed grape leaves down with an inverted heatproof plate large enough to fit the pan to keep the stuffed grape leaves from unravelling during cooking. Combine the hot reserved chicken and meat broth, lemon juice, oil, hot water and salt and pepper in a small bowl. Pour this mixture into the pan along the sides, just enough to cover the stuffed grape leaves. If necessary, add more hot water. Bring to a boil over medium/high heat. Cover, reduce the heat to low and simmer for about 40 minutes or until the filling has cooked. Shake the pan from time to time so that the cooking liquid circulates. Add a little hot water as necessary to keep the stuffed grape leaves covered. Add the cornstarch mixture to the sauce and cook for another 10 minutes, swirling the liquid around for the sauce to seep through evenly. Test one stuffed leafto ensure the leaves have softened and the filling is cooked. Taste and adjust the cooking liquid for salt and lemon juice.