How to Make It
Set a fine-mesh sieve over a medium bowl. In a heatproof medium bowl, whisk together the egg yolks, lemon zest and juice, salt, and 1 cup of the sugar. Set the bowl over (not in) a saucepan of simmering water and cook, stirring constantly, until the lemon mixture becomes opaque and has thickened slightly, 12 to 15 minutes.
Pour the lemon mixture through the sieve into the bowl. Place a piece of plastic wrap directly on the surface of the lemon mixture and refrigerate until completely cool, at least 2 hours.
Using an electric mixer, whip the cream and the remaining ½ cup of sugar on medium until soft peaks form, 2 to 3 minutes. In 3 additions, gently fold the whipped cream into the lemon mixture. Pour into an 8-by-4-inch loaf pan or another 6-cup pan, cover, and freeze until firm, at least 4 hours.
Meanwhile, heat oven to 375° F. Spread the almonds on a rimmed baking sheet and toast, tossing once, until golden brown, 4 to 6 minutes. Serve the semifreddo sprinkled with the almonds.