Remove mascarpone cheese from refrigerator and let sit at room temperature for 15-20 minutes, but no longer. See note #4 below.
In a medium sized sauce pan add the lemon juice, water and granulated sugar. Heat over medium/high heat just until the sugar has dissolved. Remove pan from heat and let lemon syrup cool for at least 10 minutes.
Meanwhile, using either a hand held or stand mixer add mascarpone cheese to mixing bowl along with lemon zest. Beat on medium low speed until just incorporated.
To the mascarpone mixture add the lemon curd (or Lemon Zabaglione Custard). Beat on medium low speed until fully incorporated making sure to scrape down the sides of the bowl halfway through beating.
Once the mascarpone mixture is smooth, slowly beat in 1/4 cup limoncello. If a pudding like consistency is not achieved, beat in an additional 1/4 cup limoncello.
In a separate bowl, using a clean beater(s), beat together whipping cream and powdered sugar on medium to medium high speed until soft peaks form, about 6-7 minutes. See note #5 below.
Fold whipped cream into mascarpone mixture and set aside.
Mix together lemon syrup and 1 cup limoncello in a shallow baking dish. Dip the lady fingers in the mixture for about 10 seconds per cookie. The cookies should be wet but not saturated.
Arranged the lady fingers in one layer on the bottom of a 9 inch springform pan or a 9 x 13 inch rectangle pan. See note #6 below.
Spoon 3-4 scoops of the mascarpone filling on top of the cookies and spread evenly over cookies.
Repeat the process with another layer of lady fingers and mascarpone filling.
Refrigerate for at least 6 hours or overnight.
Decorate with thin lemon slices before serving.