Make the candied zest

Cup of Yum
1. Remove the zest from the lemons with a zester, making sure not to remove any of the bitter white pith. Place the zest in a small bowl, cover with boiling water, and let stand for 30 minutes. Drain.
2. Bring the sugar and cold water to a boil in a small saucepan over medium-high heat, stirring to dissolve the sugar.
3. Add the drained zest, reduce the heat to medium-low, and cook for 10 minutes. Transfer the zest and syrup to a bowl, cover, and let stand overnight.
4. The next day, transfer the zest and syrup to an airtight container. (The zest can be stored in the refrigerator for up to 2 weeks.)

Make the lemon curd

1. Prepare an ice bath and set aside.
2. Whisk together the lemon juice, sugar, eggs, egg yolks, and butter in a large heavy saucepan and cook over medium heat, whisking constantly (do not walk away), until it begins to boil and becomes thick and pudding-like, 10 to 15 minutes.
3. Strain the curd through a fine-mesh strainer into a bowl, and set the bowl in the ice bath. Stir the curd periodically until it is cool, then remove the bowl from the ice bath and place a sheet of plastic wrap directly on the surface of the lemon curd. Refrigerate for at least 1 hour. (The curd can be refrigerated for up to 3 days.)
4. Whip 1 1/2 cups of the chilled heavy cream in a large mixing bowl with a wire whisk or electric mixer until soft peaks form. Remove the chilled lemon curd from the refrigerator, and set aside 1/2 cup for the sauce. Gently fold the remaining lemon curd into the whipped cream until well combined to create a soft lemon mousse. Refrigerate the reserved lemon curd. (It can be refrigerated, tightly covered, for up to 1 week.)

Assemble the semifreddo

1. Arrange eight 3-inch ring molds or 6-ounce ramekins on a parchment-lined baking sheet. Spoon the lemon mousse into the molds, filling each with about 3/4 cup. Cover and transfer to the freezer to firm up for at least 4 hours. (The mousse can be frozen for up to 1 week.)
2. Drain the candied lemon zest, reserving the syrup for the sauce, and set the zest aside on a plate or sheet of parchment.
3. Whisk 1/3 cup of the lemon syrup with the remaining 1/2 cup lemon curd and the lemon juice in a small bowl; set the sauce aside.
4. To serve, remove the ring molds or ramekins from the freezer and let them warm up for 4 to 5 minutes. (If using ramekins, you can dip each one briefly in hot water to release the semifreddo.)
5. Meanwhile, whisk the remaining 1 cup chilled cream in a medium bowl until soft peaks form. Unmold the mouse onto individual dessert plates. Spoon the lemon sauce around the mousse, top with the whipped cream, and finish with a garnish of candied lemon zest.