Soak the lamb fries overnight in milk to cover in the refrigerator, or use something like ham brine, from our recipe for smoked testicles.
Poaching
Before the testicles can be cooked, they should be gently poached and trimmed of the outer membrane. To do it, get a small pot large enough to fit the testicles snugly, fill half full with water, bring to a boil, turn off the heat, and add the testicles. Set a timer for 10 minutes. After ten minutes, remove the testicles to a cutting board and pat dry.
Trimming
Cut the end off of each testicle. Some testicles are sold with this already removed.
Remove the outer sheath from the testicle to reveal the inner, thin membrane.
With a sharp knife, cut a shallow slit in the inner membrane.
Gently pull off the inner membrane, doing your best to keep the testicle intact.
When you're finished, you should have a soft piece of meat left over. From here the testicles are ready to cook. Discard the trim.
Video
Notes
Many testicles will not have the outer membrane. If yours doesn't, skip to poaching and removing the inner membrane.