To make the topping: Mix flour, brown sugar, white sugar, cinnamon, nutmeg, and salt together in a bowl. Add butter and mix with your fingers or a fork until coarse crumbs form. Stir in walnuts.
To make the apple filling: Whisk sugar, cornstarch, ginger, and cinnamon together in a small bowl.
Place apples in a slow cooker. Stir in cornstarch mixture. Add lemon juice and toss to combine. Sprinkle topping over apples. Cover and cook until apples are tender, on Low for 4 hours or High for 2 hours.
Turn off the slow cooker, partially uncover, and let stand until topping has hardened, about 1 hour.
