Prepare the beef by cutting the beef sirloin into bite-sized pieces and season with salt, pepper, paprika, and dried thyme in a large mixing bowl.
Add the baby potatoes to the bowl and toss to combine all ingredients evenly.
In a small saucepan, melt the unsalted butter over medium heat. Once melted, remove from heat and stir in the minced garlic and Worcestershire sauce.
Pour the garlic butter mixture over the beef and potatoes in the mixing bowl. Stir well to ensure everything is coated.
Transfer the beef and potatoes mixture into the slow cooker, spreading it out evenly.
Cover the slow cooker and set it to cook on low for 6 hours or high for 3 hours, until the beef is tender and the potatoes are cooked through.
Once cooked, carefully remove the lid and stir the mixture gently. Taste and adjust seasoning if necessary.
Serve hot, garnished with freshly chopped parsley.