Butterfly each turkey tenderloin and pound it to an even thickness of 1/4-inch. Mix 2 cups of apple cider and cider vinegar. Add tenderloins and cover. Set aside to marinate in the refrigerator. Mix croutons, apples, pecans, thyme, salt, pepper, garlic, cider, and heavy cream together. Allow to macerate 10 minutes then mix again. Spread stuffing atop each tenderloin, leaving 1-inch of space along the edges. Tightly roll each tenderloin and tie with butcher's twine. Heat oil in roasting pan and sear tenderloins until lightly brown. Finish cooking tenderloins in a preheated 350-degree oven until the internal temperature reaches 170 degrees. Remove the tenderloins from the pan and allow to rest. Deglaze roasting pan with wine and remaining cider; reduce by 1/2. Reduce the heat and stir in garlic, pepper, cream, water, sage, and turkey base. Mix well and when just below a simmer, thicken with the roux. Continue to cook until smooth and thicken. Slice tenderloins into turkey medallions.
