Preheat the oven to 350°F (180°C), setting the rack in the center position. Prepare a large pot of salted water and bring it to a boil.
Separate 12 to 15 large leaves from the cabbage head and blanch them in the boiling water for about 3 minutes. Drain thoroughly and set aside.
In a medium mixing bowl, combine the ground beef, cooked rice, chopped onion, egg, milk, 1 1/2 teaspoons of salt, and black pepper. Mix thoroughly and divide into 12 equal portions.
Lay a cabbage leaf flat and place approximately 2 1/4 ounces of the beef mixture at the center. Roll the leaf around the filling, folding the edges in to secure it.
Arrange the cabbage rolls seam-side down in a large baking dish or an oven-safe Dutch oven, securing each roll with toothpicks if needed.
To prepare the sauce, mix the diced tomatoes (with juice), tomato sauce, sugar, vinegar, and 1/2 cup of water in a bowl. Pour the sauce over the cabbage rolls.
Cover the baking dish with a lid or aluminum foil and bake in the preheated oven for about 1 hour, or until the cabbage is tender and the sauce is bubbling.
Remove the toothpicks from the rolls and transfer them to a serving platter.
Pour the cooking juices from the baking dish into a saucepan and place it over medium heat. Dissolve the cornstarch in 1/4 cup of cold water, then gradually add the slurry to the saucepan, stirring constantly. Bring to a boil, reduce to a simmer, and continue to stir until the sauce reaches the desired thickness.
Serve the cabbage rolls hot, topped with the thickened tomato sauce.