Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish or line it with parchment paper for easy removal.
In a large bowl, combine the flour, rolled oats, brown sugar, baking powder, and salt. Stir until well mixed.
Pour in the melted butter and mix until the mixture resembles coarse crumbs. Reserve 1 cup of this mixture for the topping.
Press the remaining mixture evenly into the bottom of the prepared baking dish to form the crust.
In another bowl, toss the diced apples with lemon juice, cinnamon, nutmeg, and granulated sugar until the apples are well coated.
Spread the apple mixture evenly over the crust in the baking dish.
Sprinkle the reserved crumb mixture over the apples evenly.
Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and the apples are tender.
Allow the bars to cool in the pan for at least 15 minutes before slicing into squares.