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Heat oven to 200C/fan 180C/gas 6. Grease a roasting tin (30x20cm) with butter, dust with a little flour, then set aside. Peel, core and slice the apples into rings, then toss in a little lemon juice to stop them going brown.
Tip the cream and butter into a saucepan, bring up to the boil, then set aside. Whisk the sugar with the eggs until they thicken and turn pale, about 3 mins. Whisk the buttery cream into the eggs, then fold in the flour until completely smooth.
Pour the batter into the prepared tin and arrange the apple slices over the top. Scatter over the blackberries, then sprinkle with the remaining sugar. Bake for 50 mins-1 hr until golden and beginning to pull away from the sides of the tin. Leave to cool in the tin and serve cut into squares.