Preheat the oven to 350°F. Spray a 9x9 or 8x8 baking pan with baking spray.
Peel the apples with a paring knife or vegetable peeler. Cut them in half, remove the core with a "V" cut, then cut each quarter into 3 slices. Toss apple slices with sugar in a medium bowl and set aside.
5 Granny Smith apples, 2-4 tablespoon sugar
Put oats, brown sugar, flour, cinnamon, and walnuts (if using) in a food processor. Cut cold butter into ½ inch chunks and scatter on top. Combine ingredients with 6-8 1 second pulses, or until mixture is crumbly and forms large crumbs when pinched with fingers. Don't overpulse.
1 cup rolled oats, 1 cup brown sugar, ½ cup flour, ½ teaspoon cinnamon, ½ cup walnuts, ½ cup (1 stick) butter
Pour apples into prepared pan. Sprinkle crumble topping evenly over the top. Bake for 30-40 minutes, or until the crisp is golden brown and bubbling on the edges.
Remove from oven and allow to cool in the pan for 5 minutes. Serve warm with vanilla ice cream.