Pat the beef dry with paper towels and sprinkle it evenly with salt and pepper.
In a large heavy-based pot or skillet, heat 1 tablespoon of oil over high heat. Add the beef in a single layer (avoid overcrowding) and brown it for about 4 minutes on all sides. Remove the beef and repeat with the remaining pieces, adding more oil as needed.
Allow the pot to cool slightly, then return it to the stove. Melt half of the butter, then add the minced garlic and sliced onion. Cook for about 3 minutes until softened.
Sprinkle the flour over the onion and garlic mixture, stirring to combine. Add the Dijon mustard and mix well; it may look gluey, but that’s okay!
Gradually pour in about half of the beef stock while stirring to dissolve the flour mixture. It will thicken quickly. If needed, switch to a whisk to ensure a lump-free consistency. Add the remaining stock, stir well, scraping the bottom of the pot, and bring to a simmer.
Transfer the mixture to a slow cooker, add the beef, and cook on LOW for 8 hours or HIGH for 5 hours.
In a large skillet, melt half of the butter over high heat. Add half of the mushrooms and cook until they are almost golden, about 3 minutes. Add half of the minced garlic, salt, and pepper, and cook until golden. Remove and repeat with the remaining butter, mushrooms, and garlic.
In a bowl, mix the sour cream with 1.5 cups of liquid from the slow cooker. Gently stir this mixture into the stew, being careful not to break up the beef. Add the sautéed mushrooms and stir gently.
Ladle the beef stroganoff stew over wide egg noodles, pasta, or mashed potatoes. Garnish with fresh chives and enjoy!
Notes
No alcoholic drinks allowed, please substitute with a non-alcoholic option.