2 1-pound pork tenderloins, trimmed
Kosher salt and freshly ground pepper
Vegetable oil, for the grill
3 firm-ripe peaches, halved, pitted and cut into wedges
1/2 cup dry white wine
1/3 cup packed light brown sugar
2 teaspoons whole-grain mustard
1/4 teaspoon chopped fresh rosemary, plus 1 or 2 small sprigs
Preheat a grill to high. Pat the pork dry and season with salt and pepper. Lightly oil the grill, then grill the pork, turning, until marked, about 10 minutes.
Meanwhile, combine the peaches, wine, brown sugar, and 1/2 cup water in a saucepan. Remove strips of zest from the lemon with a vegetable peeler and add to the pan; squeeze in the lemon juice. Cover and cook over medium-high heat until the peaches are just tender, 5 to 7 minutes. Transfer to a bowl with a slotted spoon.
Add the mustard, 1/2 teaspoon salt and 1/4 teaspoon pepper to the juices in the pan. Transfer 1/3 cup of the liquid to a small bowl and stir in the chopped rosemary; brush onto the pork and continue grilling until a thermometer inserted into the thickest part registers 145 degrees F, 10 to 15 more minutes. Transfer to a cutting board.
Meanwhile, simmer the remaining liquid in the saucepan until syrupy, about 3 minutes. Remove from the heat and season with salt. Return the peaches to the pan along with the rosemary sprigs. Slice the pork and serve with the peaches.