Combine beef, mushrooms, cream of mushroom soup, milk, onions, and Worcestershire sauce in a slow cooker.
Cook on High for 3 to 4 hours, or on Low for 5 to 7 hours. Stir in cream cheese until well dissolved; cook for 1 hour more.
Bring a large pot of lightly salted water to a boil. Cook fusilli in the boiling water, stirring occasionally, until tender yet firm to the bite, about 12 minutes. Drain. Serve stroganoff over fusilli and garnish with sour cream.