Recipe  pilfered from some website in France, very light, foamy, strong egg and lemon taste)
Ingredients for 4 people:
- 2 lemons 
 - 3 eggs 
 - 200g sugar 
 - 1 tablespoon cornstarch 
 - 1 cup water
 
Preparation 
- Separate the whites and yolks into their own bowls. Beat the whites, then the yolks.
 - To the egg yolks add sugar. Whisk until the mixture becomes light colored and foamy.
 - To the yolk-sugar mixture, add the cornstarch and water, then add the lemon juice and zest.
 - Cook almost to the boil over low heat, whisking constantly. When the consistency resembles that of the cream, stop cooking and let cool. 
 - Beat egg whites until firm, fold into the lemon mixture with a wooden spoon, not a whisk.
 - Pour the mousse into 4 glasses and refrigerate at least 2 hours before serving cold.
 
Note: I've used 1/2 and I've used 1/4 that amount of sugar and it was just fine.