8 oz. (about 1 c.) very cold crème fraîche or sour cream
1/4 c. confectioners' sugar
8 oz. very cold heavy cream
1/4 c. shelled unsalted pistachios, chopped
2 tbsp. fresh lime juice
1 tbsp. honey
4 ripe peaches, thinly sliced
1/4 c. small fresh mint leaves
Line an 8 1⁄2"-x-4 1⁄2" loaf pan with plastic wrap, leaving an overhang on all four sides.
Using an electric mixer with the whisk attachment, beat the crème fraîche until soft peaks form, 1 to 2 minutes. Add the sugar, whisking to combine.
With the mixer on low speed, gradually beat in cream. Increase speed and beat until stiff peaks form, 2 to 3 minutes.
Fold in pistachios, then transfer to the prepared pan. Freeze until set, at least 4 hours.
Ten minutes before serving, make the peach topping: In a medium bowl, whisk together lime juice and honey. Add peaches and toss to coat. Let sit, tossing occasionally, until peaches begin to release their liquid, at least 5 minutes.
Invert the semifreddo onto a serving platter and remove the plastic. Fold the mint into the peaches, then spoon the mixture and any juices over the semifreddo.