Preheat the oven to 350°F, and coat an 8”-square pan with nonstick cooking spray.
To prepare the streusel topping, whisk together the oats, flour, and cinnamon in a small bowl. Make a well in the center. Pour in the maple syrup and melted butter. Stir until fully incorporated.
To prepare the filling, toss the apples with the cornstarch, cinnamon, and nutmeg in a large bowl until completely coated.
Transfer the filling to the prepared pan, and gently press down with a spatula. Sprinkle evenly with the topping. (The topping tends to clump, so try to break it up into fairly small pieces.) Bake at 350°F for 50-60 minutes or until the apples are fork tender. Cool completely to room temperature; then refrigerate for at least 3 hours before serving to allow the juices to fully thicken.
*Honey or agave may be substituted for the pure maple syrup.