Step 1
Preheat the oven to 170°C/fan oven 150°C/Gas Mark 3.

Step 2
Put the lemon juice into a 1.5 litre (2 ½ pint) baking dish and add the apples and blackberries, tossing to mix. Sprinkle with the sugar. Bake in the oven for five minutes whilst making the topping.

Step 3
Sift the flour, baking powder and salt into a mixing bowl and add the sugar, Trex, eggs, lemon zest and vanilla. Beat with a wooden spoon for one to two minutes until smooth and creamy.

Step 4
Remove the dish from the oven and spoon the mixture evenly over the fruit. Return to the oven and bake for 40-45 minutes until risen and golden brown. Serve with custard, cream or ice-cream.