- Preheat your Fornetto or conventional oven to 170℃ (350℉).
- Position an oven rack at the bottom of the oven if using the Fornetto baking stone or in the centre, if using a metal baking tray.
- Peel, core, and slice the apples into 1/4-inch slices. Place apples in a large mixing bowl and toss with sugar, lemon juice, flour, and cinnamon. Set aside.
- In another large bowl, mix together flour, oats, sugar, cinnamon, and salt for the topping. Using your fingers, gently work in the cold butter until pea-sized lumps are formed. Stir in the nuts.
- lightly grease the baking surface and roll out the sheet of puff pastry over top. Fill with the apples bringing them closer to the edge of the tray leaving plenty of room at the sides of the pastry. Fold the remaining pastry back inwards and over the apples leaving a large open surface of apples in the centre.
- Brush the pastry with beaten egg and sprinkle with brown sugar. Allow setting, about 5 minutes.
- Top the entire centre of the tart over the apples with the crumble mixture.
- Bake until apples are bubbling and topping is golden brown, about 45 minutes, rotating once halfway through cooking.
Tip: Serve warm or at room temperature with vanilla ice cream and caramel sauce.