Preheat oven to 200ºC (400ºF). Place the apple, rhubarb, lemon juice, sugar, cinnamon and vanilla in a bowl and toss to combine. Divide the mixture between 2 x 14cm frying pans with a heatproof handle and cover with aluminium foil. Place over medium heat and cook for 6–8 minutes or until just tender. Remove from heat. Combine the cornflour and water, divide between pans, and stir through the apple mixture. Cut 2 x 20cm rounds from the pastry and make a small cut in the top of the pastry. Place on top of the apple mixture and pinch the edges to seal. Brush the pastry with egg. Combine the granulated sugar and extra cinnamon and sprinkle over the pastry. Bake for 25–30 minutes or until golden and crisp. Serve with custard. Serves 4–6.