If you're cooking over a campfire, set up your campfire and let the flames die down to create hot coals. Place the Dutch oven on a stable surface, such as a fireproof cooking stand or a bed of hot coals. If you're making this at home, you can use your regular oven and preheat it to 375°F.
In a large bowl, combine the thinly sliced apples with lemon juice, granulated sugar, ground cinnamon, and ground nutmeg. Toss the apples until they are well coated with the spices and sugar.
In a separate bowl, mix the rolled oats, all-purpose flour, brown sugar, cold diced butter, and a pinch of salt. Use your fingers or a pastry cutter to blend the ingredients until the mixture resembles coarse crumbs.
Grease the inside of the Dutch oven with a little butter or cooking spray. Pour the spiced apples into the Dutch oven, spreading them out evenly. Sprinkle the crumb topping over the apples, covering them completely.
If using a campfire, place a few hot coals on the lid of the Dutch oven to create an even heat. Cook the apple crisp for about 35-45 minutes or until the apples are tender and the crumb topping is golden and crispy.
If using a regular oven, bake for the same amount of time at 375°F (190°C) until the same desired results are achieved. Remove the Dutch oven from the heat or oven and let it cool slightly. Serve the apple crisp warm with a dollop of whipped cream or a scoop of vanilla ice cream