Prep the beef: Trim any excess fat and cut the beef into bite-size chunks. Pat dry with paper towels.
Season with salt and pepper.
Optional sear: For extra flavor, heat butter or oil in a skillet over medium-high. Sear the beef in batches until browned on the edges, 2 to 3 minutes per side. Don’t crowd the pan.
Transfer to the slow cooker as you go.
Layer the base: Add mushrooms, onions, and garlic to the slow cooker. Place the beef on top.
Build the sauce: In a bowl, whisk together beef broth, Worcestershire, Dijon, paprika, and a pinch more salt and pepper. Pour over the beef and veggies.
Slow cook: Cover and cook on Low for 7 to 8 hours, or on High for 3.5 to 4.5 hours, until the beef is very tender.
Thicken: In a small bowl, mix cornstarch with water to make a slurry.
Stir it into the slow cooker. Cover and cook on High for 15 to 20 minutes until the sauce thickens slightly.
Add the creaminess: Turn off the heat. Stir in the sour cream until smooth.
Taste and adjust salt, pepper, or Worcestershire as needed.
Cook the noodles: Boil egg noodles according to package directions. Drain and toss with a little butter and salt.
Serve: Spoon stroganoff over noodles, rice, or mash. Top with chopped parsley and a crack of black pepper.