Using an electric mixer, beat the eggs with stevia, lemon zest, natural orange flavouring and slowly start adding lemon juice. Beat until you get a foamy emulsion.
Heat the sour cream in a deep saucepan and at the moment it starts boiling remove it from heat. Slowly start pouring in the eggs mixture and keep mixing with a whisk. At this point return the saucepan to very low heat and keep stirring. You will want the cream to thicken, so be patient and wait for it to get a nice creamy consistency. Don't stop stirring.
Let the cream cool down for at least 15 to 20 minutes. Take 2 tbsp out in a small glass and add extra orange flavouring to get a darker orange colour. This will serve as the decoration.
Using a ladle, pour it slowly over the frozen crust. Decorate the pie by sprinkling it with your darker orange cream. Using a wooden stick, create mandala art effects. You can see how it's done in the video tutorial.
Freeze for 1 hour or leave it in the refrigerator for at least 15 hours before serving.