Start with the plum sauce, because it takes longer than the pork. Combine the red wine, sugar, vinegar, and prunes in a pot. Cook over medium heat until the prunes simmer down and get really soft, about 20 minutes. While that is cooking, move on to the pork.
Preheat the oven to 400Â°F. Arrange the sage sprigs in a row down the length of the pork tenderloin and tie with butcherâ€™s twine to hold them in place. Season the pork all over with salt and pepper. Put a cast-iron (or regular ovenproof) skillet over medium-high heat. Coat the bottom of the pan with a little olive oil and get it almost smoking. Add the pork to the pan and sear on all sides until nicely browned and caramelized. Transfer the whole thing to the oven, pan and all, and roast the pork for 10 to 12 minutes.
Puree the prune mixture in a food processor or with a handheld blender. The prunes will thicken the sauce; season with salt and pepper. Cut the string off the pork but leave the sage leaves in place. Slice the pork tenderloin on a slight bias into 1-inch-thick pieces. Drizzle the sauce over the pork.
Edited: October 14th, 2009