1. Preheat oven to 350°F.
2. In a large bowl combine the apples, sugar and walnuts; sprinkle with Apple Pie Spice.
3. Transfer the apple mixture in a large, shallow glass baking dish ( we commend a 8x12 inch Pyrex pan). Set aside.
4. In a mixing bowl combine the oatmeal, flour, sugar, and salt;
5. Cut in the butter with your fingers or pasty cutter until topping resembles pebbles.Sprinkle the topping over on the apples.
6. Bake for 45 minutes or until topping is golden and crisp.
7. Serve with whipped cream or vanilla ice cream if desired.