Preheat the oven to 350ºF (180ºC) and have a 4 to 6 quart (4 to 6 L) cast iron Dutch oven ready.
Place the chopped apples directly into the Dutch oven. Add the lemon juice and stir. Add the sugar, flour, and cinnamon. Stir everything together until the apples are well coated. Set aside.
Next, make the oat topping. Combine the flour, brown sugar, oats, cinnamon, and salt in a bowl. Stir well to combine.
Add the butter to the oat mixture. Using your hands or a fork, mix the butter into the brown sugar and oats until it is evenly distributed. If your butter is fairly soft, the mixture will be quite smooth, like a soft dough.
Scatter the oat topping, evenly in chunks, over the top of the apples in the Dutch oven. Place the Dutch oven in the preheated oven, with the lid off, and bake until the top is golden and the apple filling is bubbling, about 55 to 60 minutes. Cool the apple crisp on a wire rack for 10 minutes before serving warm with ice cream or whipped cream.
Notes
If you don't own a Dutch oven, any oven-safe baking dish that will hold all of the apples comfortably will do. Just keep your eye on the crisp in the oven. When the topping is golden, and the fruit filling is bubbling, it's done.Choose apples that will hold their structure after baking. I like SweeTango, Honeycrisp, Red Prince, Jonagold, or Northern Spy. That being said, a balance of firmer and softer apples can also be nice. If you like a mix of both, use a softer apple like the classic MacIntosh for half of the apples in the recipe.Use fresh or bottled lemon juice.
Substitutions
You can use regular white sugar in place of the cane sugar.All-purpose flour can be substitutes for the whole wheat flour. You can also use buckwheat flour to make a gluten-free apple crisp.Instant oats can be swapped in for large flake, however, they don't have the same texture or chew as large flake rolled oats.Consider substituting 2 cups (200 g) of fresh or frozen cranberries for 2 cups of apple for a sweet-tart dessert.A little homemade chai spice makes a great substitute for some of the cinnamon in Dutch oven apple crisp.Substitute non-dairy butter to make this recipe dairy-free.