In a small saucepan, combine the water and sugar and stir over medium-high heat until the sugar is dissolved. Remove from the heat and stir in the limoncello and lemon juice.
Pour the syrup into a wide, shallow bowl and set aside.
To Make the Mascarpone Filling
In a large bowl, gently whisk together the mascarpone, lemon curd, limoncello and lemon juice. Set aside.
In the bowl of a stand mixer fitted with a whisk attachment (or with a large bowl and a hand mixer), beat the heavy whipping cream and sugar to medium peaks.
With a large metal spoon, gently fold 1/3 of the whipped cream into the mascarpone mixture, then fold in the remaining cream.
To Assemble the Tiramisu
Briefly dip both sides of a ladyfinger, one at a time, into the Limoncello Syrup (about 2 seconds per side) and arrange in a 9x13 inch (22x33 cm) pan to cover the bottom.
Spread half of the Lemon Mascarpone Filling over the ladyfingers and smooth the top.
Repeat to create a second layer of ladyfingers and mascarpone filling.
Cover the dish with plastic wrap and refrigerate for at least 4 hours, and up to 10 hours.
Top and serve
Just before serving, whip the cream for the topping to medium-soft peaks and dollop or pipe on top. Zest over fresh lemon if desired.
Serve as is! Store leftovers covered in the refrigerator for up to 2 days.
Recipe Notes
Limoncello substitutes: If you can’t find limoncello or choose not to use it, you can replace the limoncello in the syrup with ½ cup (4 fl oz/120 ml) of lemon juice and increase the sugar to ⅔ cup (5 oz/142 g).
If you prefer not to use limoncello in the mascarpone filling, omit it and increase the lemon curd to 2 cups (16 oz/450 g).
Don't rush the fridge time: Although this can be eaten 4 hours after it is assembled, for the best taste, let the tiramisu set in the fridge for at least 24 hours (and up to two days) to allow the flavors to develop.
Dip quickly: When dipping the ladyfingers in the syrup, do it very briefly—just about 2 seconds per side—or the cookies may get too soggy.
Buying ladyfingers: Purchase them online, or check your supermarket cookie aisle (they're often found with other European-style cookies), the international foods section near other Italian specialty foods, or in the bakery or baking aisle.