Spray a 9 by 5 by 3-inch metal loaf pan with nonstick spray. Line the pan with plastic wrap, allowing the excess to hang over the ends and sides.
Combine the crushed amaretti cookies and the melted butter. Put in the lined pan and press down to firm and form a crust.
Whisk 1/2 cup of the sugar, the egg yolks, lemon juice, lime juice, limoncello, and salt in a large metal bowl to blend. Set the bowl over a saucepan of simmering water (do not allow the bottom of the bowl to touch the water). Whisk the egg mixture until it is thick and creamy, and a thermometer inserted into the mixture registers 160 degrees F, about 5 minutes. Set the bowl of custard into another bowl of ice water to cool completely. Stir in the zest.
Using an electric mixer, beat the cream and remaining 1/4 cup sugar in another large bowl until firm peaks form. Using a large rubber spatula, gently fold the whipped cream into the custard. Spoon the mixture onto the prepared crust. Fold the overhanging plastic wrap over the custard and freeze until frozen, at least 8 hours or up to 3 days.
Unfold the plastic wrap. Invert the semifreddo onto a platter and peel off the plastic wrap. Cut the semifreddo into 1-inch slices and sprinkle with crushed amaretti cookies and serve.
“I substituted ginger snaps for the amaretti cookies and served Sweet Cherry Sauce on the side.” ~ Carol