Pat beef dry then sprinkle with all the salt and pepper.
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.
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.
Stir in flour, then mustard.
Add the beef stock and stir well, scraping the bottom of the pot, and bring to simmer.
Slow cooker: Transfer all liquid into slow cooker. Add beef, then slow cook 8 hours on LOW or 5 hours on HIGH. Pressure cooker / instant pot: 40 minutes on high, same method as slow cooker. 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.
GARLIC BUTTER MUSHROOMS: 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 mushrooms.
FINISHING STEW: 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!
* Only if you're planning to transfer to slow cooker! If cooking on stove, must use pot. 1. Beef - use economical stewing cuts that need to be cooked for a long time to become tender such as: Chuck Boneless beef ribs* Gravy beef or other beef labelled as "stewing beef" Beef cheeks** * Use ribs if you want to show off. It is the most well marbled all throughout, not a speck of dry anywhere (unlike chuck where there are some spots that are less juicy than others) ** If 200g/7oz or less, cook whole so it looks like this beef cheeks recipe but with Stroganoff Sauce. Chopped cheeks twist and skew once slow cooked - looks better to serve whole (or at most, halved). Other beef cuts ok to use - great for slow cooking but not as juicy all the way through as there are more lean patches: Brisket Blade roast Best to buy big piece and cut your own. Try not to buy the tiny little pre cut stewing beef - they will cook too quickly. 2. Sour Cream - mixing with sauce from stew prior to adding into pot/slow cooker ensures you don't end up with sour cream specks in the sauce. You can't mix stew vigorously because the beef will fall apart! 3. Instant Pot - if you have an Instant Pot (or the Australian Breville Fast-Slow Cooker) you can do the sautéing in the IP. Note that it will take longer though because it doesn't get as hot as using stove and you can't use as large a pot/pan! 4. Sauce thickness - stove method might need a touch of extra water to thin sauce, depends how heavy your pot lid is (heavier = less evaporation).