Step 1: Pat the beef cubes dry with paper towels. Season generously with salt and pepper.
Step 2: Heat olive oil in a large skillet or Dutch oven over medium-high heat. Brown the beef in batches until a nice crust forms on all sides. This step is crucial for flavor development. Remove beef and set aside.
Step 3: In the same skillet, add the chopped onion and cook until softened, about 5-7 minutes. Add minced garlic and cook for another minute until fragrant.
Step 4: Transfer the browned beef, cooked onion, and garlic to a 6-quart (or larger) slow cooker.
Step 5: Add the sliced mushrooms, cream of mushroom soup, cream of celery soup, beef broth, Worcestershire sauce, dried thyme, and paprika to the slow cooker. Stir everything gently to combine.
Step 6: Cover and cook on LOW for 6-8 hours or on HIGH for 3-4 hours, until the beef is fork-tender.
Step 7: About 30 minutes before serving, or once the beef is tender, taste the sauce and adjust seasonings if necessary (add more salt, pepper, or Worcestershire if desired).
Step 8: Just before serving, stir in the sour cream until well combined and heated through. Do not boil after adding sour cream, as it may curdle.
Step 9: Serve the Crockpot Beef Stroganoff with Creamy Mushrooms hot over cooked egg noodles, mashed potatoes, or rice. Garnish with fresh chopped parsley.