For the raspberry puree: Add the raspberries and sugar to the bowl of a food processor. Puree until smooth. (You can also do this using a stick blender.) Push the raspberry puree through a fine-mesh strainer and discard the seeds. If using, stir in the raspberry liqueur. Refrigerate until needed. (You can prepare the raspberry puree up to a day ahead of time.)
Line a 9 x 5-in (23 x 13 cm) loaf pan with two strips of parchment paper, one coming up the short sides of the pan, the second one overlapping at the bottom, but coming up the long sides of the pan. (You can also line the pan with plastic wrap but the sides of the semifreddo once unmolded won’t look as smooth.) Set aside.
For the pistachio semifreddo: First, soak the pistachios. Bring a small pot of water to a boil. Add the shelled pistachio nuts and simmer for two minutes. After two minutes, drain and rinse the nuts under cold water to cool them completely.
Dry the nuts with paper towels. Peeling the pistachios is optional but using the nuts with the skin on will change both the color and texture of the pistachio semifreddo (it won’t be as smooth). To peel a pistachio, simply pinch it between your fingers: the skin should come right off. This step takes a bit of work, but it's worth it. Alternatively, you can look for peeled pistachios, which are slightly more expensive than regular pistachios, but are going to save you some time. (It is still necessary to blanch peeled pistachios).
Once the pistachios are blanched, peeled, and cooled, transfer them to the bowl of a food processor with ¼ cup (50 g) of the sugar. Pulse until the nuts are very finely ground, stopping from time to time to scrape down the bowl. Set aside.
In a clean, stainless steel bowl, add the egg whites. Beat until foamy, then add in ½ cup of the remaining sugar one tablespoon at a time, beating about 30-45 seconds between each addition and repeating until you’ve incorporated all the sugar. Keep beating until the meringue is shiny and forms sift peaks that ever so lightly droop at the top.
In a separate bowl, beat the heavy cream with the remaining sugar (¼ cup) and the vanilla extract until soft peaks form. Using a spatula, fold the meringue into the whipped cream, then incorporate the ground pistachios.
To assemble: In the prepared pan, spread 1/3 of the pistachio semifreddo over the bottom of the pan. Drizzle with 1/3 of the raspberry puree, then sprinkle 1/3 of the chopped pistachios and 1/3 of the raspberries.
Repeat these steps once, then finish by spreading the remaining pistachio semifreddo mixture as the final layer. Level out the top and carefully fold the overlaying parchment paper over the semifreddo to protect it. (Save the remaining raspberry puree, chopped pistachios, and raspberries for serving.) Freeze for at least 6 hours or preferably overnight. Pistachiosemifreddo will keep fresh in the freezer for up to 2 weeks.
SERVING: Take the semifreddo out of the freezer. Carefully peel the parchment paper covering the top of the semifreddo. Slide a thin knife between the parchment paper and a side of the pan and run the knife all around the semifreddo to loosen it. Invert the pan over a serving dish, then lift it to unmold the semifreddo. Carefully peel off the parchment paper and discard.
Drizzle the semifreddo with the remaining raspberry puree, then garnish with pistachios and raspberries. Serve right away, using a serrated knife to slice thick pieces for each guest. Return any extra semifreddo to the freezer right away.
ADDITIONAL SERVING IDEA: You can create scoops of this pistachio semifreddo, just as you would serve ice cream. It will look beautifully rippled and be just as amazingly delicious. Serve it in bowls with extra raspberry puree, pistachios, and fresh raspberries, or over an ice cream cone.
STORAGE: Wrap leftover semifreddo with plastic wrap, or transfer to an airtight container. Enjoy within 2 weeks.
NOTE: This recipe uses a French meringue, which means the egg whites are not cooked before they’re added to the pistachio semifreddo mixture. If you’re worried about using raw egg whites, you can use liquid egg whites(egg whites sold in cartons), which are pasteurized.
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