Preheat oven to 400°F. Evenly salt the pork tenderloins and set aside for 10 minutes to absorb salt. Meanwhile, Add peach jam, grainy mustard, Dijon, and lemon zest to a bowl, and mix together. Remove 1/3 of mixture into a separate bowl, and add in chopped rosemary.
Heat 1 tbsp of oil in a large heavy skillet that is oven-safe. Roll pork tenderloins in freshly cracked pepper to get a good coating around, then place tenderloins into the hot skillet. Brown each tenderloin on all sides.
Glaze the pork tenderloins with the rosemary peach mustard. Then transfer the skillet to the oven, and cook for around 20 more minutes, or until the internal temperature reaches 145°F. Remove from oven and allow to sit for 5 minutes before slicing. Serve with the other 2/3 of peach mustard on the side.