Step 1Place the cabbage in a pot and pour boiling water over it. Cover and cook over medium heat for 10 minutes. Drain well. Carefully separate the leaves of the cabbage to obtain 12 large leaves.
Step 2Cook the beef bones, tomatoes and chopped onions in a heavy saucepan over medium heat, 20 minutes.
Step 3Meanwhile, mix the beef, grated onion, 1 1/2 teaspoons of the salt, the pepper, rice and egg until blended. Place a heaping tablespoon of the mixture on each cabbage leaf. Tuck the opposite sides in and carefully roll up the cabbage leaf. (If necessary, fasten each end with toothpicks.)
Step 4Carefully place the rolls in the tomato mixture with the remaining 1 1/2 teaspoons of salt. Cover and cook over low heat for 1 hour. Add the lemon juice and sugar. Cook uncovered until the sauce tastes sweet and sour and the sugar is dissolved, 30 minutes. Taste and correct the seasoning. Add additional lemon juice or sugar if necessary. Serve hot.