Pat the cubed stew beef completely dry with paper towels, trimming any large pieces of fat but keeping any well-marbled fat. Season the beef cubes with garlic salt and ground black pepper. Sprinkle the flour over the beef and toss to evenly coat each piece. Heat 2 tablespoons olive oil in a large skillet over medium-high heat. Sear the beef in batches for about 45 seconds per side, browning the outside without cooking through. Use tongs to transfer the browned beef to a plate. Add more olive oil between batches as needed and lower the heat if the pan becomes too hot.

In the same skillet, reduce the heat to medium and add 1 tablespoon of butter. Pour in the white wine and use a spatula to deglaze the pan, scraping up any browned bits from the beef for extra flavor. Add the diced onion and sliced mushrooms. Cook for about 4 minutes until the vegetables begin to soften. Stir in the minced garlic and cook for another minute. Remove the pan from the heat.

Transfer the beef broth, beef bouillon base or cube, red wine vinegar, Worcestershire sauce, and Dijon mustard to the slow cooker. Stir to combine thoroughly. Add the seared beef (including any juices that accumulated on the plate) along with all of the sautéed mushrooms, onions, and garlic from Step 2. Cover and cook on high for 4–5 hours, or on low for 8–9 hours, until the beef is fork-tender.

Whisk together the cornstarch and cold water in a small bowl until smooth, then pour this slurry into the slow cooker and stir well. Let it cook for a few more minutes to thicken the sauce. Switch the slow cooker to its warm setting. In a separate bowl, combine the sour cream with 1 cup of hot cooking liquid from the slow cooker and whisk until smooth, then gently stir this mixture back into the slow cooker. If you like a richer, creamier sauce, you can add the canned cream of mushroom soup at this stage and stir to combine. I like to swirl in a little extra cold butter at the end for a silky, velvety finish—just add the remaining 2 tablespoons and let them melt in.

Meanwhile, cook the wide homestyle egg noodles according to package directions in a large pot of salted boiling water until al dente. Drain well. Serve the rich beef mixture over the hot noodles, or if you prefer, you can add the cooked noodles directly into the slow cooker and combine gently before serving. For extra flavor, sometimes I like to garnish the finished dish with a sprinkle of fresh parsley or a few cracks of black pepper.