Boneless lamb tenderloins are a specialty item, so ask your butcher to prepare these for you to save the work of boning racks of lamb. The meat benefits from a very flavorful marinade, so a sauce is redundant. If you have a barbecue grill and weather permits, grill the lamb for the best flavor; otherwise, broil the meat.When you serve the dish, setting the rosemary sprigs alight is a dramatic touch that will infuse your dining room with a perfume redolent of Provence.
In a medium bowl whisk together the wine, oil, garlic, Herbes de Provence, and salt. Pour the marinade into a glass (or other non-reactive) baking dish or container just large enough to hold the lamb.
Rub all surfaces of the lamb tenderloins with the mustard. Transfer the lamb to the marinade, cover the pan with plastic wrap, and refrigerate at least 6 hours or up to 2 days.
Remove the tenderloins from the marinade and discard the marinade. Let the lamb sit at room temperature for 1 hour before cooking to remove the chill from the meat and allow it to cook more evenly. Grill or broil the lamb to your desired degree of doneness.
Let the meat rest for 5 minutes before slicing each tenderloin on a slight diagonal. Arrange the slices among 4 warm plates, fanning the slices attractively. Set the rosemary alight, if using, and place a sprig on top of each serving. Serve immediately.