Pat beef dry then sprinkle with all the salt and pepper.

Heat 1 tablespoon oil in a large heavy based pot or skillet over high heat. Add beef in a single layer and brown for about 4 minutes. Remove and repeat with remaining beef, adding more oil as needed.

Let the pot cool slightly then return to stove. Melt half the butter, then cook garlic and onion for 3 minutes until softened.

Sprinkle the flour across the surface, then stir in the mustard. Cook until it looks gluey, then gradually stir in half the stock until dissolved.

Add remaining stock, stir well, and bring to a simmer.

Slow Cooker: Transfer all liquid into slow cooker, add beef, and slow cook on LOW for 8 hours or HIGH for 5 hours.

Stove: Add beef into pot, cover with lid, and simmer gently for 2 hours.

Instant pot: Skip flour, and cook for 40 minutes on high.

Other pressure cookers: Cook for 40 minutes on high.

Melt half the butter in a large skillet over high heat. Add half the mushrooms and cook until almost golden, adding garlic, salt and pepper until golden. Repeat with remaining ingredients.

Mix sour cream with 1.5 cups of liquid from slow cooker, then gently stir into the stew.

Gently stir in mushrooms.

Serve over noodles, pasta, or mashed potato, sprinkled with chives!