Step 1: Season the beef cubes generously with salt and black pepper. In a large skillet or Dutch oven, heat the olive oil over medium-high heat. Sear the beef in batches until browned on all sides. This step is optional but adds a depth of flavor to your slow cooker beef stroganoff recipe. Remove beef and set aside.
Step 2: Add the chopped onion to the skillet and sauté until softened, about 5-7 minutes. Add the minced garlic and sliced mushrooms, cooking for another 3-5 minutes until the mushrooms have released their liquid and are lightly browned.
Step 3: Transfer the seared beef, sautéed onions, garlic, and mushrooms to your slow cooker. Pour in the beef broth, Worcestershire sauce, Dijon mustard, and dried thyme. Stir to combine.
Step 4: Cover the slow cooker and cook on LOW for 6-8 hours or on HIGH for 3-4 hours, until the beef is fork-tender.
Step 5: About 30 minutes before serving, whisk together the cornstarch and cold water in a small bowl to create a slurry. Stir this slurry into the slow cooker. Cook for another 30 minutes on HIGH until the sauce has thickened.
Step 6: Turn off the slow cooker. Gently stir in the sour cream until fully incorporated. Do not boil the sauce after adding sour cream, as it may curdle.
Step 7: Serve the creamy slow cooker beef stroganoff recipe over hot, cooked egg noodles. Garnish with fresh parsley if desired.