Start by making an X-shaped shallow cut on top of the cabbage.
Next, place it in the oven bag and add the hot water.
Close the bag and let the cabbage soak for 10 minutes.
After this time has passed, remove the softened leaves and set them aside.
Now, in a large bowl, add the ground beef, the egg, the cooked white rice, the mushrooms chopped into smaller pieces, the tomato chopped into small cubes, the breadcrumbs, the salt, the black pepper and mix all the ingredients together very well until combined.
After that, line an egg carton with plastic wrap.
Place one of the cabbage leaves we prepared in one of the holes, using it as a mold.
Fill it with one teaspoon of the meat mixture, add a slice of mozzarella cheese and cover it with one more teaspoon of the meat mixture.
Close the leaf tightly and repeat this process with all the holes in the carton.
Next, on top of each one, add one teaspoon of tomato sauce and transfer to a non-stick baking tray.
Take the baking tray to a preheated oven at 180 ° C (350 ° F) and let the cabbage balls bake for 40 minutes.
Serve.