In a 9- by 13-inch casserole, mix sugar, cinnamon, mace, and cloves and spread evenly.
Rinse pears, cut in half lengthwise, core, and stem. Lay pears, cut side down, in a single layer on sugar mixture. Dot with butter and pour orange juice evenly over fruit.
Bake in a 350° oven until sauce is bubbling and pears are hot, about 20 minutes.
Transfer pears to shallow bowls and spoon sauce over them. Top each portion with a dollop of yogurt and a dusting of nutmeg.