Preheat oven to 350 degrees F. In a cast iron dutch oven, melt butter over medium-high heat. Season beef with salt and pepper and sear until brown on all sides. Remove from pot and set aside. Add onions, celery and garlic and sauté five minutes, or until vegetables are wilted. Add flour and, using a wire whisk, stir constantly until roux is achieved. Should black specks appear, discard and begin again. Add beef broth and meat and bring to a rolling boil. Reduce heat and simmer until beef is tender, approximately 20 minutes. Add potatoes and mushrooms and cook an additional 10 minutes. Add oysters and parsley and adjust seasonings to taste using salt and pepper. Spoon mixture into 2 (9-inch) pie crusts. Place pies on a large baking sheet and bake in oven for 35 minutes until golden brown. Serve while hot and bubbly.