Zest and juice the lemons. Soften the gelatine in cold water. Combine the yoghurt with the lemon juice and zest. Whip the cream together with 3 dessert-spoons of sugar, and the egg white with 2 dessert spoons of sugar. Finally, squeeze the gelatine well, dilute in a little cream and stir into the yoghurt and lemon. Add the whipped cream and egg white slowly. Transfer to the fridge and leave to set for a few hours (at least 4).
Chop the strawberries finely and combine with the lemon juice and sugar. Chop the apple finely and drizzle with lemon juice. Arrange on plates and top with a quenelle of mousse. If desired, garnish with fresh mint.