In a large saucepan, combine white sugar, 1/4 cup flour, and salt. Stir in cranberries and maple syrup. Cook over high heat, stirring constantly.
When mixture comes to a boil, reduce heat, cover, and simmer 5 minutes, stirring occasionally. Stir apples into simmering mixture, and continue cooking for 5 minutes or until apples are tender.
Remove from heat and stir in walnuts. Pour apple mixture into pie shell; set aside.
In medium bowl, combine bread crumbs, 3/4 cup flour, brown sugar, and melted butter. Mix well and sprinkle over apple filling.
Bake at 375 degrees for 30 minutes or until topping is golden brown. Serve warm.
