Instructions:
- Pat the beef cubes completely dry with paper towels. Season generously with salt and pepper, then toss lightly in the flour until just coated.
- Heat butter in a large skillet over medium-high heat. Brown the beef in batches until nicely coloured on all sides. Remove beef and set aside.
- Add the diced onion and sliced mushrooms to the same skillet. Sauté until the onions are softened (about 5 minutes) and the mushrooms have released their liquid and started to brown slightly. Add the minced garlic and cook for 1 minute more until fragrant.
- Pour a splash of the beef broth into the skillet, scraping up any brown bits stuck to the bottom (fond). Transfer the beef, sautéed vegetables, remaining broth, Worcestershire sauce, and Dijon mustard into the slow cooker insert.
- Cover and cook on Low for 7–8 hours or on High for 3–4 hours, until the beef is fork-tender.
- About 30 minutes before serving, prepare the slurry by whisking the cornstarch and cold water in a small bowl until smooth. Stir this slurry into the slow cooker mixture. Turn the heat setting to High (if it wasn't already) and cook for another 20–30 minutes, stirring occasionally, until the sauce has visibly thickened.
- Remove the slow cooker from the heat (or switch to the Keep Warm setting). In a separate bowl, gently whisk a ladleful of the hot sauce into the 1 cup of sour cream. Once tempered, gently stir the sour cream mixture back into the entire pot. Do not boil the sauce after adding the sour cream.
- Taste and adjust seasoning. Garnish generously with fresh parsley and serve immediately over egg noodles.