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

Brown beef - Heat 1 tbsp oil in a large heavy based pot or skillet* over high heat. Add beef in a single layer (not squished) and brown aggressively all over - 4 minutes or so. Remove and repeat with rest of beef, adding more oil as needed.

Sauté aromatics - 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.

Add flour - Sprinkle the flour across the surface, then stir. Add the mustard, and stir - it will look gluey, that's ok, it will dissolve.

Add stock - While stirring, pour in about half the beef stock then mix so the flour mixture dissolves into the liquid. It will thicken quite quickly into a gravy-like consistency.Switch to a whisk if needed, to make it lump free. Then add remaining stock, stir well, scraping the bottom of the pot, and bring to simmer. Cook using chosen method below.

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

Stove: Add beef into pot. Cover with lid and adjust heat to low/medium low so the it's simmering gently. Simmer 2 hrs (check at 1.5 hrs) until beef beef falls apart easily.

Instant pot: SKIP FLOUR, follow Note 5 Gluten Free option. 40 minutes on high, same method as slow cooker.

Other pressure cookers: 40 minutes on high, same method as slow cooker.

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

Mix sour cream with 1.5 cups of liquid from slow cooker (Note 2), then gently stir into stew - careful, the beef is delicate!

Gently stir in mushrooms.

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