Step 1: Prepare the Cabbage Leaves

1. Boil the Cabbage:
   Fill a large pot with water and bring it to a rolling boil. The pot should be big enough to submerge the cabbage without overflowing.
   Cut out the core of the cabbage and carefully peel away the outer leaves, saving the largest ones to use as wraps. These should be pliable and easy to fold.

2. Boil the Cabbage Head:
   Place the whole cabbage head into the pot, on its side, and cook for about 10 minutes. Flip the cabbage to the other side and cook for another 10 minutes, or until the leaves are soft and easy to remove.
   Once cooked, carefully remove the cabbage using tongs and let it cool on a plate. Remove any tough veins from the cabbage leaves so they roll easily.

Step 2: Make the Cabbage Roll Filling

1. Cook the Onion:
   In a frying pan, melt 1 Tbsp. of butter or vegan margarine over medium heat. Dice one onion and sauté until soft, about 5 minutes.

2. Prepare the Filling:
   While the onion is cooking, break up the tempeh or mushrooms and pulse them in a food processor until finely chopped (smaller than a pencil eraser). Combine the chopped tempeh or mushrooms with the cooked rice in a large bowl.

3. Combine the Onion and Filling:
   Once the onion is soft, add it to the bowl with the tempeh/mushroom and rice mixture. Mix well and set aside.

Step 3: Make the Tomato Sauce

1. Blend the Sauce:
   Slice the second onion and add it to a food processor along with the whole tomatoes from the cans. Blend until smooth, leaving no large chunks behind. Season with salt and pepper to taste.

Step 4: Stuff and Cook the Cabbage Rolls

1. Assemble the Rolls:
   In a large pan, melt the remaining butter or vegan margarine and pour in a layer of the tomato sauce. Begin filling each cabbage leaf with about ¼ cup of the rice and tempeh/mushroom mixture. Fold the sides of the cabbage over the filling, then roll upwards into a neat bundle.

2. Cook the Rolls:
   Place the cabbage rolls seam side down into the pan with the sauce. Pour more sauce over the rolls to cover them completely. Cover the pan and simmer on low for about 2 hours, checking occasionally to ensure the sauce doesn’t dry out. If needed, add a little tomato juice or water.

Serve your vegetarian cabbage rolls hot and enjoy!