Preheat oven to 350ºF. Place the cast iron pan in the oven to preheat.
Apple Filling
Combine the apples and lemon juice in a large bowl. Add sugar, cinnamon, nutmeg, and salt to the apples, then gently toss until everything is evenly coated.
Crumble Topping
Mix oats, flour, brown sugar, cinnamon, and 1/2 teaspoon salt. Add cold butter pieces to the oat mixture using a fork or pastry cutter. Work the butter into the dry ingredients immediately. The mixture will resemble coarse crumbs when it's ready.
Assembling the Apple Crisp
Remove the cast iron pan from the oven.
Add butter and brown sugar, mixing a small bit.
Place the apple filling into the pan, filling it evenly.
Sprinkle the topping over the apples, covering everything. Place the pan back into the oven and bake for 45-50 minutes, or until the apples are cooked through and the topping is golden brown.
Allow to cool slightly before enjoying. Serve warm with a scoop of ice cream (optional).