Add tomato sauce to saucepan over medium high heat.

Add brown sugar, apple cider vinegar, salt, and pepper.

Bring to a boil.

Reduce to a simmer. Cover and cook for 20 minutes, stirring occasionally. Set aside.

Fill a large soup pot 3/4 full of water. Place over high heat until boiling. 

Turn off the heat and add the cabbage. Place a heavy plate over the cabbage, so it's fully submerged in the hot water. Cover and let soak for 30 minutes. 

Cook rice according to instructions, but replacing the water with whole milk.

Meanwhile, add 2 tablespoons of butter to a large pan over medium heat. 

When butter is foaming, add onions and garlic. Sautée until translucent, 5-7 minutes. 

Add mushrooms and thyme. Cook until the mushroom's water has evaporated and the mixture is browning, about 10 minutes.

Add remaining two tablespoons of butter. Let melt and stir to incorporate. Turn off heat.

Combine your mushroom mixture with the cooked rice.

Season liberally with salt and pepper. Set aside. 

Remove your cabbage from the pot. Peel and dry each leaf. They should be soft and malleable. 

Select one leaf. Place ~2 tablespoons of the rice mixture on the surface closest to you. 

Fold into a dumpling: Fold the sides and then tightly roll the leaf away from you. Place the dumpling seam side down in a single layer within a casserole dish. 

Repeat until you've finished the stuffing. Reserve unused cabbage leaves.

Optional: Refrigerate one hour up to overnight, to let the flavors marry. 

Preheat the oven to 350 degrees. Pour the tomato sauce over your rolls. 

Place extra cabbage leaves across the top of the sauce in a single layer.

Bake, covered, for 1.5 hours. Uncover, and bake for 30 minutes more.