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 aggressively all over for about 4 minutes. Remove and repeat with rest of beef, adding more oil as needed.

Let the pot cool down a bit 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. Add the mustard, and stir until it looks gluey.

While stirring, pour in about half the beef stock and mix until the flour mixture dissolves into the liquid. Add remaining stock, stir well, scrap the bottom of the pot, and bring to simmer.

Transfer all liquid into slow cooker. Add beef, then slow cook for 8 hours on LOW or 5 hours on HIGH.

Melt half the butter in a large skillet over high heat. Add half the mushrooms and cook until almost golden, about 3 minutes. Add half the garlic, salt, and pepper, cook until golden. Remove and repeat with remaining butter, mushrooms, and garlic.

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

Gently stir in mushrooms.

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