Preheat the oven to 350°F (175°C).
Peel, core, and slice the apples into even pieces.
In a bowl, mix the apples with lemon juice, brown sugar, cinnamon, nutmeg, and flour. Spread the mixture into the baking dish.
In another bowl, combine oats, chopped pecans, brown sugar, cinnamon, and nutmeg for the topping.
Cut in cold butter using a pastry cutter or hands until the mixture forms coarse crumbs.
Sprinkle the oat topping over the apple mixture.
Bake for 45-50 minutes, or until the topping is golden brown and the apples are tender.
Let cool slightly before serving with ice cream, whipped cream, or extra caramel sauce.