Wash the beef and pat it for drying with a paper towel before seasoning it with pepper and salt.
In a large and heavy base pot, add 1tbsp butter and adjust the heat to high
Add beef to the pot and toss it to brown for about 4 minutes. Make sure to do it in batch and do not overlayer beef slices on each other in the pot.
Cool down the pot and put it back on flame with half of the butter.
Let the butter melt, and then add onion to it. Sauté the onion until it becomes soft for about 3 minutes
Add flour to the pot and stir it after adding mustard. It will turn into a paste; mix it well.
Keep stirring the pasta and add almost half the beef stock. Stir it until the paste mixes well with the broth and gives it a good consistency.
Add rest of the stock to the pot and if there are any lumps in there, move to whisk mode for some time.
Once it’s all mixed well, let it simmer and choose the ideal slow cooking method.
Add beef and all stew to the slow cooker and cook it on low settings for 8 hours or high settings for 5 hours.
You can add beef to the pot and cover the lid on the stove. Let it simmer on low heat for about 2 hours and check if the meat pulls apart easily; otherwise, leave it for some more time.
Add butter, mushrooms, and garlic for buttery garlic mushrooms in a pan. Let them toes over high heat settings, and once done, remove from heat or pan.
Once your beef is all tender, you can set up the stew consistency by adding sour cream to the stew from the cooker. Mix it well and add to your stew.
Gently add the mushrooms to the stew and serve with noodles, pasta, or mashed potatoes.