Directions

1. Stack cabbage leaves in a large Dutch oven and pour the boiling water over top. Cover, set over high heat, and boil, rotating top leaves to the bottom. Cook for 4 to 6 minutes, until cabbage is just tender enough to be pliable, but not too soft that it tears. Drain the leaves on a work surface and let cool.
2. Heat oil in a large nonstick skillet over medium-high heat. Add the onion and salt, cook stirring often for about 6 to 8 minutes, until the onion is soft and starting to brown. Add the beef, mushrooms, and pepper.  Using a wooden spoon to break up the meat, cook for about 3 to 4 minutes until the beef is browned and no pink remains. Add tomato paste and coat the mixture. Add the vinegar, syrup and Worcestershire sauce and bring to a simmer, stirring often. Remove from heat.
3. Spoon 1/4 cup of the filling onto each cabbage leaf and roll tucking in the sides. Arrange the rolls in the skillet with seam side down. Pour the tomato sauce over rolls. Cover and cook over medium-low heat. Cook for about 20 minutes. Uncover and add the mozzarella cheese. Cover and cook for about another 2 minutes., until cheese melts. Let sit for about 10 minutes before serving.