1 whole chicken (3 1/2- to 4-pounds), backbone removed and butterflied
3 tablespoons grated lemon zest plus 1/3 cup juice from 3 lemons
1 teaspoon sugar
Salt and pepper
2 cups low-sodium chicken broth
1 cup plus 1 tablespoon water
1 teaspoon cornstarch
3 tablespoons unsalted butter
1 tablespoon finely chopped fresh parsley
SEASON Adjust oven rack to middle position and heat oven to 475 degrees. Pat chicken dry with paper towels. Combine lemon zest, sugar, and 1 teaspoon salt in small bowl. Following photo 2 (of step-by-step), rub 2 tablespoons zest mixture under skin of chicken. Season chicken with salt and pepper and transfer to roasting pan. (Seasoned chicken can be refrigerated for 2 hours)
ROAST Whisk broth, 1 cup water, lemon juice, and remaining zest mixture in 4-cup liquid measuring cup, then pour into roasting pan. (Liquid should just reach skin of thighs. If it does not, add enough water to reach skin of thighs.) Roast until skin is golden brown and thigh meat registers 170 to 175 degrees, 40 to 45 minutes. Transfer to cutting board and let rest 20 minutes.
MAKE SAUCE Pour liquid from pan, along with any accumulated chicken juices, into saucepan (you should have about 1 1/2 cups). Skim fat, then cook over medium-high until reduced to 1 cup, about 5 minutes. Whisk cornstarch with remaining water in small bowl until no lumps remain, then whisk into saucepan. Simmer until sauce is slightly thickened, about 2 minutes. Off heat, whisk in butter and parley and season with salt and pepper. Carve chicken and serve, passing sauce at table.