Cook the zabaglione
In a bowl over a pot of gently simmering water (you can also use a double boiler) add the egg yolks, 1/4 cup sugar and 1/2 cup limoncello. Cook whisking continously, until the mixture is pale yellow in color and forms ribbons when dropped on the sides of the bowl. This might take from 5 to 10 minutes.

Make the lemon syrup
In a saucepan over medium heat add the 1 cup of limoncello, 3/4 cup lemon juice, 1 cup of water and 1/2 cup sugar.  in a saucepan. Bring to a boil while stirring to dissolve the sugar. Cook for 5 minutes, turn off heat and let cool completely.

Make the Mascarpone cream and beat the egg whites
In a large bowl add the Mascarpone cheese and using a wooden spoon, stir in the lemon zest. Beat using an electric mixer until smooth and creamy. Set aside.

In another bowl add the egg whites and 1/4 cup sugar and beat until it forms firm peaks.

When the zabaglione is cooled, fold it in, in 3 to 4 batches, into the Mascarpone cream using a plastic spatula. Do not stir or fold vigorously or you will destroy the fluffy texture of the cream.

Repeat the same with the beaten egg whites, folding them in batches into the zabaglione and Mascarpone mixture.

Make the Tiramisu
Pour some of the lemon syrup into a shallow dish. Roll each ladyfinger in the lemon syrup, then place it in a serving dish (the serving dish should hold about 20 ladyfingers on a single layer). Do not oversoak or the ladyfingers will fall apart. Place ladyfingers, one next to the other until you cover the bottom of the serving dish. Top with half the Mascarpone mixture and use a spatula to spread it evenly.

Repeat with a second layer of ladyfingers dipped in the lemon syrup, followed by the remaining Mascarpone cream mixture.

Cover with plastic wrap and refrigerate for at least 6 hours (best if refrigerated for 24 hours).