Slice the tenderloins lengthwise, sprinkle with salt and pepper. Put the chopped prunes and apples down the middle of one half of each tenderloin. Place other halves on top, tie with string to hold them together.
Put in a shallow roasting pan, preferably on a rack. Bake 1 1/2 hours or until a meat thermometer registers 170. Remove to a cutting board to rest. Keep warm with a tent of aluminum foil.
Put the 3/4 cup water in the roasting pan, stir to dislodge the yummy brown bits from the bottom of the pan.
Make a slurry of the 1/4 cup cold water and the flour, gradually whisk it into the pan. Simmer until thick, at least one minute. Season with the salt and pepper, as desired.