Preheat the oven to 450°F. Coat a roasting rack and roasting pan with nonstick spray.
Wash the chicken and pat dry with paper towels. (Reserve the giblets for another use.) Season the cavity with 1 teaspoon of the rosemary and the salt. Place the lemon inside the cavity. Place the chicken, breast side up, on the prepared roasting rack. Rub 1 teaspoon of the remaining rosemary over the breast meat under the skin of the chicken. Scatter the onion around the bottom of the pan. Pour in 1 can of the broth.
Roast the chicken for 20 minutes, basting with the pan juices. Lower the heat to 350°F. Roast, basting every 15 minutes, for 45 to 50 minutes, or until a thermometer inserted in a breast registers 180°F and the juices run clear. Transfer the chicken to a cutting board and loosely cover with foil.
In a small bowl, whisk the cornstarch with 1/4 cup of the remaining broth until smooth. Set aside.
Place the roasting pan on the stove top over medium-high heat. Add the wine. Boil for 2 to 3 minutes, scraping the bottom of the pan to remove the browned bits, or until reduced to 1/4 cup. Add the remaining broth. Bring to a boil. Skim off and discard any fat that rises to the top. Pour in the reserved cornstarch mixture and the remaining 1 teaspoon rosemary. Cook, stirring constantly, for 2 to 3 minutes, or until slightly thickened.
Carve the chicken. Remove and discard the skin before eating. Serve with the gravy.