Remove mushroom stems; chop stems. Sauté mushroom caps, chopped stems, and onion in butter in a large skillet until vegetables are tender. Stir in salt and pepper. Reduce heat; cover and simmer 5 minutes.
Combine flour and milk, stirring well. Stir milk mixture into mushroom mixture; cook over medium heat until bubbly. Remove from heat; add sour cream and Worcestershire sauce, stirring well. Cook over low heat until thoroughly heated. Serve hot over toast points and garnish with parsley.