In a saucepan, heat 2 cups of water, 1 tablespoon of sugar, and the peel of 1 whole lemon over medium heat, stirring constantly until the sugar dissolves.
Turn off the heat and stir in the limoncello. Set the soaking liquid aside and allow it to cool completely before refrigerating.
Make the lemon cream:
In a different medium saucepan, whisk together the egg yolks, sugar, and lemon zest until the mixture becomes frothy and thick.
Place the saucepan over low heat and continue stirring with a hand whisk as you gradually add the fresh lemon juice.
Increase the heat to medium and continue stirring until the mixture is a thick and smooth lemon cream.
Transfer the lemon cream to a bowl and let it cool.
Make the lemon tiramisu filling:
Pour the heavy whipping cream into a chilled mixing bowl and then add the powdered sugar. Now, whip the cream with an electric mixer until stiff peaks form and it has turned into light and fluffy whipped cream.
In a different large bowl, whisk the mascarpone cheese until it’s fluffy. Then add the lemon cream and whisk until fully combined. The lemon cream must be cool before you add it to the mascarpone.
Now, use a rubber spatula to very gently fold the whipped cream into the lemon mascarpone mixture.
Transfer the lemon filling into a piping bag fitted with a round tip. This will make it easier to distribute the filling and create a decorative pattern. If you don’t have a piping bag you can use a freezer bag or even just a spoon.
Put the lemon filling in the fridge until ready to use.
Assemble the lemon tiramisu:
Pour the limoncello soaking liquid into a shallow bowl. Then dip half of the ladyfingers, one at a time, into the liquid and then place each one in a single layer on the bottom of a deep 8” dish. You want to moisten both sides without making them soggy. You may need to break some of the cookies in half to cover the whole dish.
Sprinkle some of the fresh lemon zest over the soaked ladyfingers.
Then top the layer of ladyfingers with dollops of the lemon filling, making sure to cover all the cookies.
Dip the other half of the ladyfingers in the soaking liquid and place them in the baking dish to create a second layer of cookies.
Sprinkle the second layer with some fresh lemon zest and then add a second layer of lemon cream.
Put the tiramisu in the fridge to chill for at least 2 hours before serving.