Preheat your oven to 350°F (175°C) and position a rack in the center. Lightly grease a 9×9-inch baking dish or 2-quart casserole dish with butter or cooking spray.
In a large bowl, toss the sliced apples with lemon juice to prevent browning.
In a separate small bowl, whisk together the sugar, flour, cinnamon, nutmeg, and salt.
Sprinkle this mixture over the apples and toss until each slice is evenly coated.
Combine the oats, flour, brown sugar, chopped hazelnuts, cocoa powder, and salt in a medium bowl.
Using your fingertips or a pastry cutter, work the cold butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining.
Fold in the chocolate chips.
Transfer the apple filling to your prepared baking dish, creating an even layer. Gently press down to eliminate large air pockets.
Sprinkle the chocolate hazelnut topping evenly over the apples, covering the fruit completely.
Bake for 40-45 minutes, until the topping is crisp and the filling is bubbling around the edges. If the topping starts browning too quickly, loosely cover with aluminum foil for the remaining bake time.
Remove from the oven and let stand for 15 minutes to allow the filling to set slightly.
If desired, warm 2 tablespoons of hazelnut spread in the microwave for 20 seconds and drizzle over the top before serving.