1. Preheat oven to 420 degrees F (220 degrees C).
2. Rinse turkey breast and pat dry with paper towels. Place skin side up into a roasting pan.
3. Squeeze 1/2 lemon over turkey, season inside with salt and pepper, stuff with squeezed lemon, the remaining lemon half, onion halves and parsley stems.
4. Separate the skin from the turkey breast by pushing your fingers under the skin. Be careful not to tear it. Insert butter pieces under the skin (it will keep the breast tender and juicy).
5. Sprinkle with salt, pepper and parsley on all sides.
6. Roast in the preheated oven for 25 minutes. Remove and baste the breast with the juices from the bottom of the dish.
7. Cover the breast with foil, reduce heat to 350 degrees F (180 degrees C), return to the oven and cook for another 1 1/2 hours, or until cooked through. Remove foil during the last 10 minutes to get a crispy skin.
8. Let rest for at least 15 minutes. Serve.