I am not a huge fan of red meat but this Slow Cooker Beef Stroganoff is to die for! The only problem is finding porcini mushrooms. They are difficult to come by but you can find them at natural food stores such as Sunflower market if you have one nearby. It is definitely worth the trouble.
Combine 1/2 cup broth and porcini in bowl and microwave until steamy and mushrooms have softened, about 1 minute. Line fine mesh strainer with one paper towel and strain mushrooms, reserving liquid. Chop porcini fine and set aside.
Heat oil in large skillet over medium-high heat until shimmering. Cook onions and tomato paste, stirring frequently, until lightly browned, 7 to 10 minutes. Stir in wine, soy sauce, remaining broth, chopped porcini, and reserved porcini liquid, scraping up any browned bits with wooden spoon. Bring broth mixture to a boil, then transfer to slow cooker. Add beef and white mushrooms to slow cooker, cover and cook on high until meat is tender, 6 to 7 hours (or cook on low for 9 to 10 hours).
Set slow cooker to high, if necessary. Skim fat from surface. Transfer 2 cups sauce from slow cooker to large bowl and whisk in flour. Stir flour mixture into slow cooker and cook, covered until sauce thickens, about 15 minutes. Stir in sour cream and dill and season with salt and pepper. Serve over buttered egg noodles or rice.