Prepare the Beef:
Season the beef with garlic salt and black pepper. Toss with flour until evenly coated.
Heat olive oil and 1 tablespoon butter in a large skillet over medium-high heat.
Sear the beef in batches until browned on all sides (about 2-3 minutes per side). Transfer to the slow cooker.
Sauté the Vegetables:
In the same skillet, add another tablespoon of butter and sauté the onions and mushrooms until soft.
Stir in the garlic and cook for another 30 seconds. Transfer to the slow cooker.
Deglaze the Pan:
Pour white wine into the skillet, scraping up any browned bits. Let it simmer for 1-2 minutes.
Add this mixture to the slow cooker along with beef broth, bouillon, red wine vinegar, Worcestershire sauce, and Dijon mustard.
Slow Cook:
Cook on LOW for 6-8 hours or HIGH for 4 hours, until the beef is fork-tender.
Thicken the Sauce:
In a small bowl, whisk together cornstarch and cold water. Stir into the slow cooker.
Let cook for an additional 20-30 minutes until thickened.
Finish the Dish:
Stir in sour cream and cream of mushroom soup (if using).
Cook for another 10 minutes until well combined.
Serve & Enjoy:
Cook the egg noodles according to package instructions.
Serve the stroganoff over the noodles and enjoy!