Prepare your loaf pan by lining with plastic wrap completely.
Take half of the pistachio and roughly chop them. Set aside with a handful separately for decoration.
Take the other half of the pistachios and grind them in a food processor with the tablespoon of sugar until it starts to become oily and starts turning into a paste.
Combine the ground pistachios with the heavy cream in your mixing bowl. Using the whip attachment for your stand mixer or the beaters for your hand mixer, whip on medium-high speed until soft-medium peaks.
If you don't have a second mixer bowl, transfer the whipped cream to a new bowl and set aside in the fridge. Thoroughly clean the bowl and attachments before making the meringue.
In a small pot combine the second measurement of granulated sugar with just enough water to make wet sand, and in your mixer bowl add the egg whites.
Using the whip attachment for your stand mixer or the beaters for your hand mixer, start whipping the egg whites on medium speed.
At the same time as above step, bring the sugar water mixture to a boil and continue to cook.
While the sugar is cooking and the egg whites are whipping, constantly check back and forth between the two. The egg whites should be no more than a soft peak and the sugar should be cooked to 245℉. If the egg whites are getting too stiff too fast, reduce the speed to the lowest speed. If the sugar is cooking too fast, either increase the whipping speed of the whites or splash a little water in the pot. If the sugar syrup overcooks then add a little more water.
When your whites are at soft peaks your sugar is 245℉ remove from heat and reduce the whipping to medium-low. Slowly drizzle the sugar mixture down the side of the bowl into the whipping egg whites. When all the sugar is in the bowl, increase the speed to medium-high and allow to whip the Italian meringue until full, stiff 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. Fold in another one-third of the meringue and fold it in two-thirds of the way.
With the last third of the meringue, fold two-thirds of the way again then add in the pistachios (again reserving a handful for decoration) and half of the chocolate sauce. Fold until the pistachios and chocolate are 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.
Release the semifreddo from the edges of the pan; you may want to use hot water or a blowtorch. Flip over onto your plate and remove the pan and plastic wrap.
Microwave the remaining chocolate sauce for 15-20 seconds and drizzle on top followed by the reserved pistachios. Allow to firm up in the freezer for at least 15 minutes before serving. Serve when ready to eat.