Preheat your oven to 375°F (190°C) and lightly grease an 8x8-inch or 9-inch round baking dish.
In a large mixing bowl, toss the sliced apples with 3/4 cup granulated sugar, 1 tablespoon flour, cinnamon, and nutmeg until the apples are evenly coated. Pour the apple mixture into the prepared baking dish, spreading it into an even layer.
For the streusel topping, combine the rolled oats, 1 cup flour, brown sugar, baking powder, and salt in a medium bowl. Use a pastry cutter or your fingers to cut the cold, cubed butter into the oat mixture until it resembles coarse crumbs with some pea-sized pieces of butter remaining.
Sprinkle the streusel topping evenly over the apple mixture, covering it completely.
Bake for 40 to 45 minutes, or until the topping is golden brown and the apple filling is bubbly around the edges.
Remove the crisp from the oven and allow it to cool on a wire rack for at least 15 minutes before serving to allow the filling to set.