In a large stockpot or saucepan (8-quart or larger), fry the bacon over medium high heat for three minutes, or until the fat has rendered and the bacon is golden. Drain the bacon fat, leaving about 2 tablespoons, or enough to coat the bottom of the pot. Add the beef and pork or venison, stirring constantly for about five minutes, or until all sides of the meat are browned. Stir in the onion, carrots, and mushrooms and sauté the vegetables until they are slightly caramelized, about another 3 minutes.
In a small bowl, dilute the tomato paste with ½ cup of the beef stock, adding the stock slowly and stirring constantly.
Still over high heat, add the rest of the beef stock, scraping the bottom of the pot to deglaze it. Slowly stir in the tomato paste mixture, and then the bay leaves, sauerkraut, cabbage, and prunes or wine. Season with the salt and pepper, tasting the stew and adding more if needed. Cover the pot and bring the stew to a boil. Reduce the heat to medium-low and simmer for one hour, stirring occasionally to prevent sticking.
Stir in the kielbasa. Cover and simmer the stew over low heat for about 30 minutes. Serve the stew immediately, or reheated after it’s had a chance for flavors to develop for a day or so in the refrigerator.