Place the turkey breast in a small roasting pan and season with salt and pepper, inside and outside.
Place 2 onion halves and 2 garlic cloves with 1 stalk of rosemary inside/underneath of the breast cavity.
Place the other onion halves and garlic and 2 other rosemary stalks around the turkey breast in the pan. Pour in the 2 cups of stock. (I added stock because the turkey breast will not release as much liquid as a whole turkey will. If you are cooking a whole turkey, you can leave the stock out. The stock will give enough liquid to make a gravy after it has cooked.)
Roast at 325 for 2 1/4 hours (or as directed by lb.) or until internal thermometer reads 165. Add glaze 2 times during the roasting.
Once the turkey is in the oven, begin make the glaze and reducing the liquids. To a small saucepan, add the balsamic vinegar, maple syrup and chopped rosemary leaves. Bring to bubbling and bubble over medium-low heat for about 30 minutes, stirring often. Watch the video to see how you know the glaze is finished.
Add about 1/3 of the warm glaze onto the turkey. Pull off the heat and let cool. After another hour in the oven, add the next 1/3 round of glaze to the turkey. Once the turkey is cooked and cooled for about 15 minutes, add the last of the glaze before slicing and serving.