Toast the pistachios over a low heat for 2-3 minutes until fragrant.
Soak the cherries with the orange zest, juice, and rum for at least 30 minutes, 40 is ideal. Drain the juice from the cherries and set aside.
Line a metal loaf pan with parchment paper, followed by cling film. Make sure it is snug, and there is plenty of extra parchment and cling film arising from the loaf pan sides since you need to cover the Semifreddo when it goes into the freezer.
In a mixer on medium/low, beat the egg yolks and sugar until creamy. Add the salt, followed by the soaking liquid that you have reserved. Continue mixing until the consistency is even throughout.
Add the chopped cherries and mix just until they are distributed. Set aside the mixing bowl.
Separately, whip the heavy cream until Very whipped, almost too thick but of course, pre-butter!
Add the whipped cream into the mixing bowl of cherry mix and fold gently by hand, with a rubber spatula. Keep folding as you try to integrate the two textures without knocking the air out. The mix must remain fluffy and light. Towards the end of the folding, add in the chopped pistachios and keep folding until they are well distributed.
Carefully pour the Semifreddo into the loaf pan until it reaches just under the top (I managed to use just about all it it.) Smooth out the top and gently wrap the top with the parchment and the cling film. Pop it into the freezer and check back the next morning, or six hours at the very earliest. I found it really needed overnight or even two for perfection.
You can slide the Semifreddo out of the loaf pan when ready to serve--the high sides of cling film will help here. Adorn the top with additional orange slices, cherries, and ground pistachios if you like to show off (I did!)