Peel, core, and slice the apples into thin wedges, about ¼ inch thick. Place them in a large bowl and toss with lemon juice to keep them from browning.
Add the granulated sugar, cinnamon, nutmeg, and flour to the apples, and toss to coat the slices evenly. Spread the apple mixture evenly into a greased 9x13-inch baking dish.
In a medium bowl, combine the flour, oats, brown sugar, cinnamon, and salt. Cut the cold butter into the dry ingredients until the mixture resembles coarse crumbs.
Stir in the sourdough discard until the topping forms clumps. Evenly sprinkle the sourdough crisp topping over the apple mixture.
Preheat your oven to 350°F (175°C). Bake the apple crisp for 40-45 minutes, or until the topping is golden brown and the apples are bubbling underneath.
Let the apple crisp cool for about 10-15 minutes before serving.
