In a saucepan, melt butter. Add mushrooms and cook gently 2 minutes. Add flour and stir into mushroom mixture.
Add cheese, mustard and Worcestershire sauce. Stir well and heat through gently 2 minutes or until mixture begins to melt. Remove from heat and season to taste with pepper.
Preheat broiler. Toast bread on one side only. Spread cheese and mushroom mixture on untoasted sides. Broil in preheated broiler 3-4 minutes or until melted and bubbling. Cut slices in half and sprinkle liberally with parsley and paprika. Garnish with parsley sprigs, if desired, and serve hot.