Preheat oven to 350F (180C).
Rinse the turkey breast with cold water. Pat dry.
Liberally salt and pepper the inside cavity of the turkey.
Cut the lemons in quarters and place two quarters in the cavity, along with the garlic. Add rosemary, thyme and sage to the inside of the turkey.
Brush the turkey with olive oil and season liberally with salt and pepper.
Place the turkey in a large shallow roasting pan lined with aluminum foil (for easy clean-up).
Add 1 cup water to bottom of pan.
Place the reserved lemons and the sliced onion in a large bowl and toss with 1 tablespoon of olive oil, 1 teaspoon salt, and ½ teaspoon pepper. Pour the lemon and onion mixture around the turkey inside the pan.
Roast the turkey breast for 2 – 2 ½ hours, or until an instant-read thermometer registers 165 degrees F. Baste with pan juices every 30 minutes.
If any part of the turkey browns too quickly while cooking, just tent lightly with aluminum foil.
Remove the turkey from the oven and place on a large cutting board. Tent the turkey with foil and allow it to rest for about 20 minutes before carving.