Prep the base: Add the sliced mushrooms and onions to the bottom of your slow cooker or Instant Pot.
Scatter the garlic on top.
Layer the beef: Place the beef over the vegetables. Season with 1 teaspoon salt, 1/2 teaspoon black pepper, and the paprika.
Mix the sauce: In a bowl or measuring cup, whisk the beef broth, Worcestershire, and Dijon. If using cream of mushroom soup, whisk that in too.
Pour everything over the beef.
Cook it low and slow (slow cooker): Cover and cook on Low for 7–8 hours or on High for 4–5 hours, until the beef is fork-tender.
Or use the pressure cooker (Instant Pot): Seal and cook on High Pressure for 35 minutes (40 for larger chunks). Natural release for 10 minutes, then quick release the rest of the pressure.
Thicken the sauce (if needed): Stir 1 tablespoon cornstarch with 1 tablespoon cold water. Set the cooker to Sauté (or turn slow cooker to High), stir in the slurry, and simmer 3–5 minutes until slightly thickened.
Repeat with another tablespoon if you prefer a thicker sauce.
Finish with creaminess: Turn off the heat. Stir in the sour cream and butter until smooth. Taste and adjust salt and pepper.
If you want more tang, add another teaspoon of Dijon.
Cook the noodles: Boil egg noodles in salted water according to package directions, usually 6–8 minutes. Drain well.
Serve: Spoon the stroganoff over the noodles. Sprinkle with chopped parsley and extra black pepper.