Pour just enough water in a double-boiler pan so the water level is right below the bottom of the mixing bowl when it is sitting in the pan. Separate the eggs, putting yolks into the large bowl of the double boiler and the whites into another stainless-steel bowl for whipping by hand or with an electric mixer.
Remove the zest of two or more of the lemons, using a fine grater, to get 2 tablespoons of zest. Squeeze out and strain the juice of these and the other lemons to get 3/4 cup of fresh lemon juice.
To make the base for the tiramisù, heat the water in the double boiler to a steady simmer. Off the heat, beat the egg yolks with 1/4 cup of the sugar and 1/2 cup of the limoncello until well blended. Set the bowl over the simmering water, and whisk constantly, frequently scraping the whisk around the sides and bottom of the bowl, as the egg mixture expands and heats into a frothy sponge, 5 minutes or longer. When the sponge has thickened enough to form a ribbon when it drops on the surface, take the bowl off the double-boiler pan and let it cool.
Meanwhile, pour the remaining cup of limoncello, all of the lemon juice, 1 cup water, and 1/2 cup of the sugar in a saucepan. Bring to a boil, stirring to dissolve the sugar, and cook for 5 minutes, evaporating the alcohol. Let the syrup cool completely.
In another large bowl, stir the mascarpone with a wooden spoon to soften it, then drop in the grated lemon zest and beat until light and creamy. Whip the egg whites with the remaining 1/4 cup sugar, by hand or by machine, until it holds moderately firm peaks.
When the cooked limoncello sponge (or zabaglione) is cooled, scrape about a third of it over the mascarpone, and fold it in with a large rubber spatula. Fold in the rest of the zabaglione in two or three additions. Now fold in the whipped egg whites in several additions, until the limoncello-mascarpone cream is light and evenly blended.
Pour some of the cooled syrup, no deeper than 1/4 inch, into the shallow-rimmed pan to moisten the ladyfingers (savoiardi). One at a time, roll a ladyfinger in the syrup and place it in the casserole or baking dish. Wet each cookie briefly—if it soaks up too much syrup, it will fall apart. Arrange the moistened ladyfingers in neat, tight rows, filling the bottom of the pan completely. You should be able to fit about twenty ladyfingers in a single layer.
Scoop half of the limoncello-mascarpone cream onto the ladyfingers, and smooth it to fill the pan and cover them. Dip and arrange a second layer of ladyfingers in the pan, and cover it completely with the remainder of the cream.
Smooth the cream with the spatula, and seal the tiramisù airtight in plastic wrap. Before serving, refrigerate for 6 hours (or up to 2 days), or put it in the freezer for 2 hours. To serve, cut portions of tiramisù in any size you like, and life each out of the pan and onto dessert plates.