In nonstick skillet, toast walnuts over medium heat until fragrant, 3 min. Transfer to plate.
In same skillet, heat oil over ned-high heat. Add onion and celery, cook 3 min. Add apple, raisins, bulgur, broth, salt and cinnamon. Bring to a boil. Lower heat to med., cover. Simmer 12 to 15 min. or until bulgur is tender and liquid is absorbed. Stir in walnuts.
Reduce oven to 375deg. Turn squash halves, cut side up. Fill each squash with about 1/2 cup stuffing. Drizzle with remaining syrup.
Bake in 375deg oven for 15 min. or until stuffing is heated through and tops are glazed with syrup.