Step 1In a blender, puree the raspberries with 1 tablespoon sugar. Taste for sweetness, adding more sugar if needed. The sauce should be tart, not overly sweet.
Step 2Strain to remove seeds. Store in the refrigerator covered for up to 5 days.
Step 1Line a rectangular loaf pan with plastic, leaving a 2-inch overhang. Chill in the freezer.
Step 2In a large metal bowl, whisk together the eggs and sugar. Place the bowl over a large saucepan of simmering water, making sure the circumference of the bowl is larger than the pot. Whisk until the sugar has dissolved and the mixture has thickened and reached 150 degrees, about 3 minutes.
Step 3Remove from the heat and continue whisking for another 3 minutes, until the mixture has cooled slightly. Set the bowl over an ice bath. Place 2 cups raspberries in a small bowl and mash lightly with the back of a fork. Add the mashed berries to the egg-sugar mixture, leaving the bowl over the ice bath.
Step 4Using an electric mixer, whip the cream to stiff peaks.
Step 5Add one cup of the whipped cream to the berry mixture and use a spoon or spatula to mix well. Gradually fold in the rest of the cream, very lightly until just incorporated.
Step 6Spoon the cream mixture into the chilled pan and smooth the surface. Sprinkle the pistachios evenly over the top. Cover lightly with plastic and freeze for at least 8 hours, or overnight. The recipe to this point may be made up to one day before serving.
Step 7Remove the semifreddo from the freezer about 20 minutes before serving. Just before serving, unmold the semifreddo onto a cutting board and peel off plastic wrap. Using a knife dipped in hot water, cut into 8 vertical slices. Use a large spoon to make a circle of raspberry sauce on each plate (about 1 tablespoon per plate). Top with a slice of semifreddo. Drizzle with another tablespoon of raspberry sauce and sprinkle with fresh raspberries.