Prepare an ice bath and set aside.
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.
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.)
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.)