Coat a 2- to 4-quart slow cooker with cooking spray; set aside.
In an extra-large bowl combine 3 tablespoons granulated sugar, lemon juice, cornstarch, ginger, and 1/4 teaspoon cinnamon. Gently stir in apples. Transfer apple mixture to the prepared cooker.
For topping, in a small bowl stir together flour, 1/4 cup granulated sugar, brown sugar, 1/4 teaspoon cinnamon, nutmeg, and salt. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Stir in walnuts. Sprinkle topping over apple mixture.
Cover and cook on low-heat setting about 4 hours or on high-heat setting about 2 hours or until apples are tender. If desired, serve with ice cream.