Step 1. Prepare the apples.
Peel, core, and slice your apples. Toss them in the cast iron skillet with lemon juice, half the maple syrup, and 1 teaspoon cinnamon. Spread evenly across the skillet.
Step 2. Mix the topping.
In a bowl, combine oats, flour, remaining cinnamon, nutmeg, and salt. Cut in the cold butter until the mixture resembles coarse crumbs. Drizzle in the rest of the maple syrup and stir until combined.
Step 3. Assemble.
Sprinkle the topping evenly over the apples.
Step 4. Bake.
Bake at 350°F for 40–45 minutes, until the apples are tender and the topping is golden brown.
Step 5. Serve.
Scoop warm apple crisp into bowls and serve plain, with whipped cream, or with a scoop of vanilla ice cream.