Line a 9x5 inch loaf pan with two sheets of plastic wrap, one in each direction, leaving at least 5 cm overhang on all sides. Press it into the corners as smoothly as you can. Set aside in the freezer.

Fill a medium saucepan with about 5 cm of water and bring to a gentle simmer. Place a heatproof bowl on top, making sure the base doesn't touch the water.

Add the 4 egg yolks and 80 g of caster sugar to the bowl. Whisk continuously for 6 to 8 minutes until the mixture is pale, thick, and falls from the whisk in a slow ribbon that holds for 5 seconds.

Remove the bowl from the heat. Whisk in the lemon juice, lemon zest, and limoncello if using. Set aside and let the mixture cool to room temperature, stirring occasionally. This step is important - a warm base will deflate the cream and whites.

Pour the cold heavy cream into a chilled bowl. Whip with a hand mixer on medium-high speed until it just holds soft peaks. It should look glossy and mound gently - not stiff or grainy. Set aside in the refrigerator.

In a clean, grease-free bowl, whip the 4 egg whites on medium speed until foamy. Gradually add the remaining 40 g of caster sugar, one teaspoon at a time, increasing to high speed. Whip until stiff, glossy peaks form and the mixture doesn't slide when you tilt the bowl.

Fold the whipped cream into the cooled lemon zabaione in two additions, using a rubber spatula with a gentle, sweeping motion from the bottom of the bowl. Keep as much volume as possible.

Add half the whipped egg whites and fold until just combined. Add the remaining whites and fold again until no white streaks remain. Work quickly but gently.

Pour the mixture into the prepared frozen loaf pan. Smooth the top with the spatula. Fold the overhanging plastic wrap over the surface to cover.

Freeze for a minimum of 6 hours. Overnight is better for a cleaner slice.

To unmold, lift the semifreddo out of the pan using the plastic wrap overhang and place it upside down on a cold serving plate. Peel off the plastic. Let it sit at room temperature for 5 to 8 minutes before slicing.

Slice with a sharp knife dipped in hot water and wiped dry between cuts. Serve immediately with raspberries, a dusting of icing sugar, or crushed pistachios.