Whisk the eggs and caster sugar in a large bowl until very thick and the mixture leaves a trail when lifted. Sift the flour over the surface of the eggs, then fold in very gently. Add the lemon rind and juice and fold in until just mixed. Pour the mixture into a greased, floured 25 cm (10 inch) sponge flan tin, tilting the tin to ease into an even layer.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 10–12 minutes until the top of the sponge is golden and the centre springs back when lightly pressed. Cool the sponge in the tin for 5–10 minutes, then carefully turn it out on to a wire rack to cool.
Whip the cream until it forms soft swirls, then fold in the fromage frais and lemon curd. Transfer the sponge to a serving plate, spoon the cream into the centre, spread into an even layer, then top with the strawberries and blueberries. Warm the redcurrant jelly in a small saucepan with the measured water (or lemon juice), then brush over the fruit.