In a large skillet, cook beef, salt, and pepper over medium heat 6-8 minutes or until no longer pink, breaking into crumbles; drain. Transfer meat to a 4-qt. slow cooker.
Add mushrooms, onions, parsley, and garlic over the top of the ground beef in the slow cooker.
Whisk together beef stock, wine, and tomato paste. Pour over meat mixture; stir to combine. Cook, covered, on low 4-6 hours.
Once the meal is ready, add the cornstarch slurry and stir to combine. The sauce will immediately start to thicken.
Stir in sour cream just before serving.
Serve with noodles, rice, or mashed potatoes, a side salad and steamed vegetables for a quick weeknight meal.
Use a good table wine - one you enjoy the flavor of as the flavor will intensify. If you do not like wine, omit it without substitution.
To substitute all-purpose flour, use 4 tablespoons flour mixed with 1/2 cup cold water. Be sure to thoroughly mix the water with the flour to prevent lumps. Stir into the cooking liquid and cook high heat until sauce thickens. Taste sauce and adjust seasonings, if needed.
Sour cream can curdle if it boils so thicken the main beef gravy first and then turn off the crock pot and finish the sauce by stirring the sour cream into the sauce right at the end before serving without further heating.