In a small saucepan cook and stir oil and garlic over low heat for 2 minutes. Remove from heat. Stir in lemon peel; set oil mixture aside.
Place sorrel leaves, if using, and lemon wedges in the cavity of the bird. Tie legs to tail. Twist wing tips under back.
Loosen and lift skin above breast. Brush a little of the garlic-oil mixture under the breast skin; skewer neck skin to back. Brush a little more of the oil mixture over the skin of the bird. Cover and chill remaining oil mixture. Cover chicken; let stand at room temperature 30 minutes, or refrigerate up to 24 hours.
Preheat oven to 375 degrees F. Uncover chicken; place, breast side up, on a rack in a shallow pan. If desired, insert a meat thermometer into center of an inside thigh muscle. Do not allow thermometer tip to touch bone. Roast, uncovered, for 1 to 1-1/4 hours or until drumsticks move easily in their sockets, chicken is no longer pink, and meat thermometer registers 180 degrees F. Baste with remaining oil mixture about halfway through cooking time. Remove sorrel leaves, if using, and lemon wedges from cavity. Cover chicken loosely with foil; let stand 10 minutes.
Meanwhile, pour juices and browned bits from roasting pan into a small glass measure. Skim off and discard fat, reserving pan juices (1 to 2 tablespoons total).
In a small saucepan gradually stir chicken broth into potato starch or cornstarch. Add pan juices. Cook and stir over medium heat until slightly thickened and bubbly. Stir in lemon juice. Season to taste with salt and pepper.
Transfer chicken to a serving platter. Pour sauce into a bowl; top with shredded sorrel or spinach. Top chicken with kale and chamomile flowers (or other edible flowers), if desired. Makes 6 servings.