Preheat oven to 350°F (180°C). Pat turkey dry with paper towels. Zest the lemons; reserve zest. Cut zested lemons in half. Remove leaves from rosemary stems and finely chop; reserve stems. In small bowl, mix butter with half of each of the lemon zest, chopped rosemary and pepper.
With fingers, gently loosen the turkey skin on breasts. Spread half the butter mixture underneath the skin. Rub the remaining butter mixture on the outside of the turkey. Sprinkle remaining pepper in the cavity. Insert the 4 lemon halves, reserved rosemary stems and onion. Tie legs together with butcher's twine. Place turkey, breast-side up, on a rack in a roasting pan. Tent with foil. Roast bird on the lowest rack 1 1/2 hr. Remove foil. Continue roasting another 1 1/2 hr.
Meanwhile, mix 1/4 cup (60 mL) wine and honey. After turkey has roasted 3 hr., baste with wine and honey mixture. Roast up to another 30 min., or until an instant-read thermometer inserted into thickest part of thigh reads 180°F (82°C).
Remove turkey from oven; transfer to serving platter. Tent with foil. Let rest 15 min. Meanwhile, pour pan drippings into a bowl. Allow oil to rise to the surface, skim off with a spoon. Pass drippings through a fine mesh strainer (yield should be about 1 1/2 cups/375 mL).
Set roasting pan on stovetop over medium-high heat. Stir in remaining white wine; scraping browned bits from bottom of pan. Add drippings and chicken broth; bring to a boil. Dilute cornstarch in 1/4 cup (60 mL) water, mixing until smooth. Whisk cornstarch mixture into dripping mixture until smooth. Bring to boil, stirring, until gravy thickens. Stir in remaining zest and chopped rosemary. Serve with turkey.