Step 1: Load the Slow Cooker
Start by adding the beef, onion, garlic, and mushrooms to the bottom of your slow cooker. Pour in the beef broth and Worcestershire sauce. Sprinkle with thyme, salt, and pepper.

Stir everything gently to combine. The beef should be mostly covered in liquid.

Step 2: Cook Low and Slow
Cover and cook on low for 7–8 hours or high for 4–5 hours, until the beef is fall-apart tender. Resist the urge to lift the lid often—keeping the heat in helps everything cook evenly.

Step 3: Add the Sour Cream
About 30 minutes before serving, stir in the sour cream. Let it melt into the sauce. If you’d like a thicker consistency, mix the cornstarch with a tablespoon or two of cold water and stir it in now.

Cover again and cook for the final 30 minutes.

Step 4: Serve and Garnish
Spoon the stroganoff over egg noodles, rice, or mashed potatoes. Garnish with fresh parsley just before serving.

You can also serve with a slice of crusty bread or a side salad for a complete meal.