Heat a 12-inch skillet over medium heat. Add 1 tablespoon oil. Add mushrooms and sprinkle with a pinch of salt. Let mushrooms cook for 5 to 10 minutes, or until softened. Pour mushrooms into a 5-quart slow cooker.
Heat remaining 2 tablespoons oil in the now empty skillet. Add onion, tomato paste, garlic, and thyme. Cook until onions have softened, stirring regularly, about 5 to 10 minutes. Stir in flour and cook for 1 minute.
Whisk in broth and wine, scraping up any browned bits. Be sure to whisk until there are no flour lumps. Pour into the slow cooker. Stir in soy sauce and bay leaves.
Season beef chunks with salt and pepper. Add beef chunks to the slow cooker, pushing them into the sauce.
Cover and cook until beef is tender, 5 to 7 hours on High or 9 to 11 hours on Low. Skim any accumulated fat from the surface. Discard bay leaves.
Add 1 cup of cooking liquid from slow cooker to a bowl. Stir in Shamrock Farms Premium Sour Cream and mustard. Return mixture to the slow cooker, stirring to combine. Season with additional salt, pepper, and dill to taste.