Thaw 2 lbs. of ground beef mixture. Bring a large pot of water to a boil. Carefully lower the head of cabbage into the water. As the cabbage simmers, the leaves will start to loosen and can be pulled off with a pair of tongs. As each cabbage leaf comes loose, remove it from the simmering water, and into a bowl of cold water. You'll need about 12 large leaves. Reserve the rest of the cabbage. When the cabbage leaves are ready, divide the beef and rice mixture into 12 logs. Place the meat at the bottom of the cabbage leaf (where the stem is the thickest) and roll up, folding in the sides as you roll. The rice will expand, so don't roll too tightly. In saucepan, stir together the beef broth, water, and tomato, and bring to a simmer over medium-high heat. In a large Dutch oven, or other deep roasting pan with a lid, cover the bottom with some of the leftover cabbage leaves. Place in 6 of the cabbage rolls. Top with the onions, and 1/2 of the hot tomato mixture. Top this layer and the last 6 rolls, and the rest of the tomato mixture. Cover with any remaining leftover cabbage leaves. If needed, add water so that the cabbage rolls are covered by about an inch of liquid. Cover and bake in a 350 degree F. oven for 1 hour 15 minutes. Remove from the oven, and use a fork to test one of the rolls to see if the rice is cooked. If it needs more time, add more water, cover, and cook for another 15-20 minutes, or until the rice is tender. When done, remove, and let rest covered for 30 minutes before serving.