How to Make It
Preheat oven to 350°F. Lightly grease a 9x13 baking dish with butter or nonstick spray.
Make the base: Toss apples, lemon juice, cinnamon, and salt in the baking dish.
Make the cake mix: Combine flour, sugar, oats, baking powder, baking soda, cinnamon, ginger, and salt in a medium bowl and sprinkle evenly over the base. Using the largest holes on a box grater, grate the butter evenly over the surface of the cake.
Make the streusel: Whisk together flour, brown sugar, granulated sugar, cinnamon, ginger, and salt in a medium bowl. Add butter and stir, squeezing between your palms, to help clumps form. Sprinkle streusel evenly over the top of the cake.
Bake until cake is golden brown, 30 to 45 minutes. Cool 30 minutes before serving.