Preheat oven to 375°F.
In a skillet, melt 2 tbsp butter over medium heat.
Add sliced apples, sugar, vanilla, and cinnamon. Cook for 5 minutes until apples begin to soften.
In a bowl, combine oats, hazelnuts, brown sugar, flour, and softened butter until crumbly.
Sprinkle the hazelnut mixture over the apples.
Transfer skillet to oven and bake for 20-25 minutes until topping is golden.
Let cool for 5 minutes before serving. Serve warm with whipped cream or ice cream.