Preheat your oven to 400°. Rub each pork tenderloin with 1 teaspoon salt each and set aside. Let stand unrefrigerated while you prepare the rest of the ingredients.
In a small saucepan, bring the apple cider vinegar, chicken stock, honey, and sage leaves to a boil. Reduce heat, and simmer until liquid is reduced to ½ cup—approximately 25-30 minutes. Remove sage leaves and set aside the reduced mixture.
Heat 1 Tablespoon oil in an ovenproof skillet over high heat until almost smoking. Sear the tenderloins on each side for 2-3 minutes. Move the skillet to the oven and roast until internal temperature reaches 145°. Remove the pork from the skillet and set aside to rest.
Return skillet to stovetop over medium-high heat, and add additional oil (if necessary) to reach approximately 1 Tablespoon of oil in the pan. Add the mushrooms, shallots, garlic, and a pinch of salt, and sauté until golden brown.
Reduce heat to medium, deglaze the pan with the hard cider, and let it reduce for 2-3 minutes. Then add the reduced vinegar mixture and mustard and reduce again for 3-5 minutes. Add the crème fraiche, chives, and parsley. Cook for an additional 2-3 minutes to allow flavors to combine. Then finish with additional salt and pepper to taste.
Slice the rested tenderloin crosswise, and serve with a generous scoop of mushroom sauce.
Adapted from food52.com