Preheat the oven to 350°F (175°C). Lightly grease a 9×13-inch baking dish with nonstick baking spray.

Place the sliced Granny Smith apples in a large mixing bowl. Add the brown sugar, flour, granulated sugar, cinnamon, nutmeg, and vanilla.

Gently toss until the apples are evenly coated with the sugar and spice mixture.

Transfer the apple mixture to the prepared baking dish. Spread it evenly without pressing or packing the apples too firmly.

In another large bowl, combine the quick-cooking oats, flour, brown sugar, cinnamon, salt, and baking powder.

Pour the melted butter over the oat mixture. Stir until everything is evenly moistened and no dry pockets of flour remain.

Sprinkle the topping evenly over the apples, making sure the fruit is well covered.

Bake the Old Fashioned Apple Crisp uncovered for 40–45 minutes, or until the topping is golden brown and the apple filling is bubbling around the edges.

Let the crisp rest for at least 5 minutes before serving. This allows the juices to thicken slightly while keeping the dessert deliciously warm.