Cut 2 lemons into 12 slices; sprinkle with 1 t. seasoning. With fingers, loosen skin from chicken breasts. Arrange 3 lemon slices under skin of each breast; smooth skin over lemons. Place in large roasting pan.
Cut remaining lemons into edges. Place lemon wedges and onion around chicken. Sprinkle with salt, pepper and remaining seasoning; add olives.
Roast until juices run clear when chicken is pierced with fork, about 45 minutes. Transfer to serving platter. Drizzle with pan juices. Let stand 10 minutes.
Just before serving, sprinkle with mint. Garnish with mint sprigs.