Heat a 5-quart stockpot of water over high heat. Stir in 2 tablespoons of Kosher salt, cover and bring water to a boil.
While waiting for water to boil, prepare an ice water bath in a large mixing bowl or the sink.
Place the cabbage leaves into the boiling salted water; boil 12 minutes or until cabbage is tender.
Remove leaves from water and place into the ice bath for 4 minutes.
Remove cabbage from bath and pat leaves dry with a paper towel.
Remove thick rib from the center of each cabbage leaf, being careful not to completely cut the leaves in half; set cabbage aside.
Slice potatoes into 1/4-inch thick slices. Place slices into a buttered 10 X 12 baking dish.
Cover potatoes with as little chicken stock as needed to cover (reserve the rest) and set aside.
Preheat oven to 350° F.
In a medium non-stick sauté pan, over medium heat, cook butter and sliced onions; add thyme and sauté 20 minutes or until onions are tender.
Remove pan from heat and season with 2 teaspoons of Kosher salt and white pepper.
Place onion mixture in an even layer over the sliced potatoes in the chicken stock. Cover dish with foil and bake 20 minutes.
Remove from oven and allow to cool.
While potato-onion mixture is baking, prepare the wild mushrooms: in a medium non-stick sauté pan, melt the butter over low heat.
Add the sliced wild mushrooms and cook for about 20 minutes over low heat until browned.
Add white wine, stir and cook until liquid has evaporated and mixture is dry. Add fresh thyme, salt and pepper. Remove from heat and allow to cool.
To assemble torte, lay a cooked cabbage leaf on a flat counter or cutting board. Place a layer of the potato and onion mixture in the center of the cabbage leaf (do not spread mixture all the way to the edges of the leaf). Top with 1/6th of the wild mushrooms.
Repeat the process 5 more times, starting with potato, then wild mushrooms, creating alternating layers. Once the layers are complete, pull the sides of the cabbage leaf up and over the top as much as possible. Lay another leaf on top and use remaining leaves to completely enclose the torte.
Lay a large piece of plastic wrap on a flat surface. Place potato and cabbage torte in the center of the plastic wrap. Take edges of the plastic and wrap around the torte, twisting the edges of the wrap. Tie wrap closed with kitchen string and set aside.
Heat reserved chicken stock in a 5-quart stockpot over medium heat. Place plastic-wrapped torte in heated chicken stock; poach for 5 minutes. Remove from stockpot and remove plastic, being careful to avoid steam when unwrapping the torte. Serve torte immediately.