Prep the beef: Trim excess fat and cut into chunks. Pat dry and season lightly with salt and pepper.
Dry meat browns better and tastes richer.
Optional browning step: Heat a large skillet over medium-high. Add butter or oil. Brown beef in batches, 2–3 minutes per side.
This adds flavor but isn’t mandatory if you’re short on time.
Layer the slow cooker: Add sliced onion and mushrooms to the bottom. Place beef on top. Sprinkle in garlic, paprika, and additional salt and pepper.
Mix the sauce base: In a bowl, whisk beef broth with tomato paste, Dijon, and Worcestershire.
Pour over the beef and vegetables.
Cook: Cover and cook on Low for 7–8 hours or High for 4–5 hours, until the beef is very tender.
Thicken the sauce (optional): If you want a thicker gravy, whisk cornstarch with water to make a slurry. Stir it into the slow cooker and cook on High for 15–20 minutes until it lightly thickens.
Add sour cream: Turn off the heat. Stir in sour cream until smooth.
Taste and adjust salt, pepper, or mustard as needed.
Serve: Spoon stroganoff over cooked egg noodles, rice, or mashed potatoes. Garnish with chopped parsley.