Thaw turkey if frozen; remove neck and giblets, rinse, and pat dry. Let turkey sit at room temperature 30 minutes.
Preheat oven to 400° F, moving rack to the lower-third section.
In a small saucepan, heat olive oil, rosemary, garlic, and zest from the lemons (about 3 tablespoons). Bring to a simmer over medium-high and cook about 1 minute until garlic is soft and lemon is slightly shriveled. Remove from heat and let cool.
Tuck wing tips underneath body of the turkey. Season inside of turkey with 1/2 the salt and pepper. Tie legs together with kitchen twine. Brush 1/2 the rosemary-lemon oil all over the turkey and season with remaining salt and pepper.
Cut onions into wedges and place in a heavy-bottomed roasting pan. Set a roasting rack over onions and place turkey on top; roast 45-60 minutes until golden brown.
Reduce heat to 375° F, add 1 cup water to the pan, and continue to roast 1.5-2 hours, brushing turkey with remaining oil mixture every 30 minutes until an instant-read thermometer inserted in thickest part of a thigh reads 165° F. (Add more water as necessary if pan becomes too dry.)