Prepare your loaf pan by lining with parchment paper down the long side allowing it to poke up above the lip of the pan.
Take half of the pistachio and roughly chop them. Set aside.
Take the other half of the pistachio and grind them in a food processor until it starts to become oily and starts turning into a paste. Set aside.
Using the whip attachment for your stand mixer or the beaters for your hand mixer, whip the heavy cream on medium-high speed until soft-medium peaks.
Lightly fold the ground pistachios (not the roughly chopped ones) into the whipped cream. If you don't have a second mixer bowl, transfer to a new bowl and set aside in the fridge.
If you do not have a second mixer bowl or attachments, thoroughly clean the bowl and attachments.
Using the whip attachment for your stand mixer or the beaters for your hand mixer, whip the egg whites on medium speed until very soft peaks.
At the same time as above, mix the sugar with enough water to create wet sand in a small pot and cook the sugar to 245℉. Make sure your egg whites do not get any stiffer than soft peaks which may mean changing the speed while checking on the sugar temperature.
With the mixer speed on medium-low, slowly drizzle the sugar mixture down the side of the bowl into the whipping egg whites. Increase the speed to medium-high and allow to whip the Italian meringue until full peaks.
Before you start folding, have everything nearby: the chocolate sauce, the whipped cream, the chopped pistachios, tools/equipment, etc.
Fold one-third of the meringue into the whipped cream and fold it two-thirds of the way. Repeat 2 more times; the meringue should be all used up.
Add in the pistachios and drizzle in half of the chocolate sauce and fold until mostly incorporated.
Pour into the loaf pan and level with the top. Place in the freezer for a minimum of 6 hours or preferably overnight.
With a wet hot knife, release the semifreddo from the edges of the long sides. Using the parchment thats sticking out, delicately lift from the pan to release. Flip over onto your plate and remove pan and parchment paper.
Serve by slicing into slices and with a drizzle of chocolate sauce and extra pistachios.