In a frying pan, saute the mushrooms in butter until tender. Blend in flour and cook until bubbly. Gradually add milk and cook until smooth and thickened, stirring constantly. Add salt, celery salt, pepper, lemon juice, and oysters. Cook over medium-low heat until oysters start to curl up, about 5 minutes, stirring occasionally.
Preheat oven to 350 degrees F.
Soften cream cheese and butter and blend well in a medium bowl. Stir in flour to form dough. Transfer dough into a plastic kitchen bag and chill in refrigerator for 1 hour. Remove from fridge and shape into 7 balls about 1/2-ounce each. Flatten and place in an ungreased 4-ounce cup muffin tin. Bake 30 to 40 minutes, or until golden.
Fill crusts with oyster filling; garnish with parsley and Parmesan. Serve hot.
Tip: For a quicker recipe, a store-bought mini-pie crust may be substituted.