Remove 10 large cabbage leaves and set aside.
Combine the rice, mushrooms, onion, and butter in a large bowl. Add the ground pork and mix everything, ensuring it’s all well combined.
Scoop some of the filling into a cabbage leave, then roll up like a burrito. Place in the pot. Continue to repeat with the remaining cabbage leaves.
Pour the can of tomato sauce in the pot. Add the salt to the empty can, then fill the can up with water, and pour that in the pot as well. Press down on the cabbage rolls a bit to try to ensure they’re at least partially covered with the sauce.
Turn on medium heat and bring the sauce to a boil. Reduce to low, cover (leave the lid ever-so-slightly ajar), and let simmer for 3 hours, or until the internal temperature of the rolls reaches at least 160 degrees. Enjoy!