Bring a large pot of salted water to a boil. Core cabbage and boil 8 to 12 minutes. Remove softened leaves as they loosen. Trim thick stems.
For the filling, saute 16 oz finely chopped mushrooms and the diced onion in 1 tablespoon olive oil or butter until soft. Cool slightly, then take ¾ of the mixture and mix with the ground beef, rice and soy sauce. The remaining quarter of the mixture is for the sauce.
In a large pan with remaining mushroom mixture. Stir in tomatoes and salt. Simmer 10–15 minutes.
Spread chopped leftover cabbage and about 1 cup sauce on the bottom of the dutch oven.
Place about ¼ cup filling on each cabbage leaf. Fold sides and roll. Arrange seam-side down in the pot.
Pour remaining sauce over the rolls, cover, and bake at 350°F for about 90 minutes until cabbage is tender and filling reaches 160–165°F.
Optional: stir a splash of heavy cream into the sauce at the end for a creamy version.
