Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9x5 inch loaf pan.
In a large mixing bowl, mix together the ground turkey, bread crumbs, onion, egg and milk. Season with balsamic vinegar, salt, pepper and rosemary. Press into the prepared pan. Blend together the tomato sauce, brown sugar and mustard; pour evenly over the top of the loaf.
Bake for 1 hour in the preheated oven, or until juices run clear when pricked with a knife.