Separate the egg yolks from the whites. Beat the yolks together with half the sugar for 10 minutes. (Making mousse is hard! Stand-up mixers make for great gifts...) Boil the milk in a saucepan for 10-15 minutes until it is reduced by half, then let it cool. Slowly add the beaten and sweetened egg yolks. Heat the preparation gently, while continuing to beat it steadily to help maintain the foam. Add a pinch of flour, while continuing to whisk. As soon as the mixture has thickened, remove the pan from the heat and let it cool. If you'd like, you can thicken the mousse with three or four leaves of gelatin, first soaked in cold water. Mix in the citrus juices and the grated orange and lemon peel. Refrigerate for 20 minutes. Meanwhile, beat the egg whites with a whisk or an electric beater to firmness. Incorporate the rest of the sugar and proceed with the whipping until you have achieved a hard and shiny consistency. Whip the heavy cream. Mix in the cooled pale yellow mixture and the egg whites. Set your ensuing orange mousse in the refrigerator for several hours before serving.