In a medium bowl, whisk together all of the ingredients for the marinade and pour over the tenderloins. Marinate the pork tenderloin for 30 minutes in the refrigerator.
While the pork is marinating, heat the oil in a skillet over medium heat and add the onions, shallots, thyme, salt and pepper. Cook until the onions are soft, about 5 minutes. Turn off the heat and set aside in the pan.
Preheat the oven to 350°F.
Remove the tenderloins from the marinade and pat them dry. Season with salt and freshly cracked pepper. Heat a large skillet over medium-high heat. Sear the tenderloins on all sides in the dry, hot skillet and finish cooking in the oven until the pork is just cooked through, about 12 to 15 minutes.
Place the tenderloins on a cutting board and let them rest 2 to 3 minutes before slicing.
Meanwhile, place the onion mixture over medium heat and add the dried plums. Cook for 1 minute and add the apple cider. Bring to a boil and cook until the cider reduces a little and becomes syrupy, about 2 to 3 minutes.
Slice the pork, place a few slices on each plate along with some of the dried plum mixture and a drizzle of the cider syrup.