1.
Grease and base-line a 20cm springform pan with nonstick baking parchment. Whisk the egg yolks and sugar with the lemon rind in a bowl over a pan of simmering water until thick and pale. Fold in the ground almonds, semolina and flour with the lemon juice.
2.
Whisk the egg whites until soft peaks form. Stir a small spoonful of egg white into the yolk mixture to lighten it, then fold in the rest. Spoon into the cake tin. Bake at 180°C (350°F) for about 20-25min or until golden, firm, and beginning to shrink slightly from the edges of the tin.
3.
Turn on to a wire rack lined with nonstick baking parchment. Cool. Carefully slice horizontally into two thin rounds. Rinse and dry the tin. Line the base and sides with nonstick baking parchment, dabbing melted butter around the edge to attach the paper. Place one half of the cake in the base of the tin.
4.
For the mousse, put 45ml lemon juice in a small bowl with 15ml water. Sprinkle on the gelatine and leave to soak until sponge-like in texture. Meanwhile, in a bowl, whisk together the egg yolks, sugar and lemon rind until thick. Gradually whisk in the remaining lemon juice.
5.
Dissolve the gelatine by placing the bowl over a pan of simmering water; whisk into the egg yolk mixture. Whip cream until it just holds its shape, then fold into the egg mixture.
6.
Whisk the egg whites until soft peaks form. Stir a small spoonful of egg white into the mousse to lighten the mixture, then gently fold in the remainder.
7.
Pour the mousse over the cake in the tin; level the surface if necessary. Cover and refrigerate until lightly set, about 45 min. Carefully place the second cake layer on top of the mousse; re-cover. Refrigerate until set for at least 4 hours, preferably overnight.
8.
To serve, remove the sides of the tin and carefully peel away the paper. Place a flat plate on top of the gateau and quickly invert. Ease off the bottom of the tin. Dust the gateau generously with sifted icing sugar and accompany with berries.