Combine the cream and sugar in a saucepan, then heat the mixture over medium heat while stirring to dissolve the sugar. Once small bubbles form on the surface, take it off the heat. Add the lemon zest and lemon juice and stir well. Although optional, you can strain the cream through a sieve to remove the lemon zest. Allow the lemon posset to cool for 10 minutes, then refrigerate for at least 2 hours before use.
Place the mascarpone cheese in a large bowl and whisk until creamy. Fold in the lemon posset and gently mix until combined and refrigerate until ready to use.
Place the sugar, water and lemon peel in a saucepan. Bring to a boil over low heat, the sugar should dissolve completely. Remove from the heat, whisk in the limoncello and let the syrup cool completely.
Transfer the limoncello syrup in a small bowl, then dip each ladyfinger for 1 second on each side.
Place the ladyfingers in a rectangular 6x8 inches baking dish, and continue the process until the whole base is covered with ladyfingers. If necessary, feel free to break the cookie in half in order to fill the base up.
Spread half of the prepared lemon mascarpone cream gently over the ladyfinger base, making sure they're evenly covered. You can use a spatula or large spoon to help you with this.
Repeat the process of soaking the ladyfingers in the limoncello syrup, then layer them over the cream, until is fully covered.
Finally, spread evenly the remaining mascarpone cream on top of the ladyfingers.
Place the Tiramisu in the fridge, and allow to set for at least a couple of hours, or overnight, then serve.