1. Soak the gelatin sheets in a bowl of cold water for 5 minutes until soft; if using powdered gelatin, bloom 1 1/2 teaspoons in 2 tablespoons of cold water for 5 minutes

2. Grate the zest of both lemons, then juice them to yield about 90ml

3. In a small saucepan, combine the lemon juice and soaked gelatin

4. Heat over low heat, stirring constantly, until the gelatin is fully dissolved—do not boil

5. Remove from heat and let cool slightly

6. In a medium bowl, beat the egg yolks with the sugar until pale and fluffy, about 3 minutes

7. Slowly whisk the warm gelatin mixture into the yolk mixture

8. Add the lemon zest and stir until incorporated

9. Let the mixture cool to room temperature, stirring occasionally to prevent a skin from forming

10. In a separate bowl, whip 480ml (2 cups) of the heavy cream to stiff peaks

11. Fold one-third of the whipped cream into the lemon mixture to lighten it, then gently fold in the remaining cream

12. In another clean bowl, whip the egg whites to stiff peaks

13. Gently fold the whites into the mousse in two additions, taking care not to deflate the mixture

14. Divide the mousse evenly among 8 serving glasses or ramekins

15. Refrigerate for at least 3 hours, or until fully set

16. Just before serving, top each mousse with a spoonful of the remaining whipped cream (120ml / 1/2 cup), a few raspberries, and a light sprinkle of lemon zest if desired