Rub tenderloin with sage and garlic salt. Place tenderloin in a baking pan or casserole. Cover meat with half of the sauerkraut. Place the apple and onion, cut side down, on top of the sauerkraut. Cover with remaining sauerkraut. Sprinkle with brown sugar.
Cover and bake in preheated oven for 2 to 3 hours, until the internal temperature has reached 145 degrees F (63 degrees C).