Place a baking tray on the lower middle rack of your oven. Heat the oven to 225°C (425°F).
Peel, quarter, core and slice the remaining 1kg of apples. Stir them through the cooled filling, then spread the filling in the chilled pastry case. Evenly top with crumb topping.
Place the pie on the hot baking tray in the oven. Bake it at 225°C (425°F) for 20 minutes, then reduce the heat to 190°C (375°F) and finish baking for 30-40 minutes, until the filling is bubbly and the pie is golden-brown. Loosely cover the top with aluminum foil if it gets too dark anytime during baking.
Cool the pie on a cooling rack for at least 30 minutes before serving, better overnight for clean slices. The pie keeps at room temperature (covered with clingfilm or a clean kitchen towel) for up to 3 days.