Peel, core, and slice the fresh apples into 1/2-inch wedges. Keep slices uniform for even cooking.
Place a cast iron skillet over medium campfire coals or a camp stove set to medium heat. Add 3 tablespoons butter and let it melt, swirling to coat the bottom.
Add the apple slices, brown sugar, cinnamon, and nutmeg (if using). Stir gently to coat. Cook for 8-10 minutes, stirring every 3 minutes, until apples are tender and slightly caramelized.
While apples cook, prepare the oat crumble: In a small bowl, mix rolled oats and flour. Cut in 2 tablespoons cold butter with your fingers or a fork until the mixture resembles coarse crumbs.
Sprinkle the oat crumble evenly over the cooked apples. Cover the skillet with a lid or foil and cook for 5 minutes more, until the topping is golden and fragrant.
Remove from heat. Drizzle with maple syrup if desired. Let cool for 2 minutes before serving. Serve warm, straight from the skillet.
Store leftovers in a sealed container. Reheat gently in a skillet over low heat.