Bring a large pot of water to a boil. Submerge the cabbage and cook until the leaves soften. Carefully remove and peel off 12 large leaves. Trim the tough veins from each leaf using a knife or kitchen scissors.

In a large skillet, melt butter over medium heat. Sauté chopped onions until translucent, then add the garlic and cook for another minute. Stir in crushed tomatoes, tomato sauce, and condensed tomato soup. Season with red pepper flakes, salt, and black pepper. Add sugar and vinegar, simmer for about 5 minutes, then remove from heat.

Preheat your oven to 350°F. Grease a 9x13-inch baking dish. Pour a thin layer of the prepared sauce into the bottom of the dish and set aside.

In a large mixing bowl, combine ground beef, cooked rice, onions, garlic, salt, black pepper, egg, and 2 tablespoons of parsley. Add 1/4 cup of the cooled sauce to the mixture and stir well.

Scoop about 1/3 cup of the meat mixture and shape it into a small log. Place each portion in the center of a cabbage leaf, folding the sides inward and rolling it tightly around the filling. Lay the rolls seam side down in the prepared dish.

Pour the remaining sauce over the cabbage rolls. Cover the dish tightly with foil and bake for 1 hour and 30 minutes, or until the cabbage is tender and the meat is cooked through.

Uncover, sprinkle with the remaining parsley, and serve hot.