Grate the zest from the lemon and place in a small bowl. Set aside the zested lemon.
Stir sumac, salt, pepper, cinnamon and allspice into the lemon zest. Stir in garlic, add small amount of olive oil if desired (I didn’t find this added much value, so I tend to it out). Rub mixture all over the chicken breasts.
In a large bowl, toss together plums, shallots, honey, oil, salt, cinnamon, allspice and bay leaf. Spread out plum mixture evenly over the bottom of a roasting pan with high sides. Lay thyme sprigs on top of plums. Place chickens over the plums in the pan and cover. Roast for 30-40 minutes, until chicken has reached an internal temperature of 165 degrees Fahrenheit. Plums should be soft.
Remove from oven and squeeze 1 tablespoon juice from reserved lemon onto chicken. Let chicken rest, covered lightly with foil, for 10 minutes. Carve and serve with the plums and more thyme for garnish.