Infuse: Bring milk and lemon zest to a boil. Remove from heat, cover, and steep for 20 mins.
Yolks: Whisk egg yolks and sugar until pale.
Combine: Reheat the milk slightly. Slowly pour hot milk into the yolk mixture while whisking.
Cook: Pour back into the pot. Cook over low heat, stirring constantly until slightly thickened (do not boil).
Gelatin: Remove from heat. Add soaked gelatin and stir until dissolved. Stir in lemon juice.
Strain: Strain the mixture through a sieve to remove zest. Let it cool to room temperature.
Whip Cream: Whip heavy cream to soft peaks (yogurt consistency).
Fold: Gently fold the whipped cream into the cooled custard base.
Chill: Pour into a lined 6-inch mold. Refrigerate for at least 4 hours.
Serve: Unmold and decorate with lime slices.