Prep the Beef: Cut the chuck roast into bite-sized cubes. Season with garlic salt and pepper, then toss with flour.
Sear the Meat: Heat oil in a skillet over medium-high heat. Sear the beef on all sides (about 45 seconds per side), then set aside.
Deglaze the Pan: Add butter and white wine (or broth) to the skillet, scraping up any browned bits. Add onions and mushrooms, sauté for 4 minutes. Add garlic, cook for 1 more minute. Remove from heat.
Slow Cook: In the slow cooker, mix beef broth, Worcestershire sauce, Dijon mustard, red wine vinegar, and bouillon. Add seared beef, sautéed onions, and mushrooms.
Cook: Cover and cook on low for 8 hours or high for 4-5 hours until beef is tender.
Thicken the Sauce: Mix cornstarch with cold water and stir into the slow cooker. Let it thicken for a few minutes.
Add Creaminess: Whisk sour cream with 1 cup of liquid from the slow cooker, then stir it into the pot. Stir in condensed cream of mushroom soup if using.
Finish & Serve: Turn off the heat and swirl in 2 tbsp butter for extra richness. Serve over egg noodles.