Preheat oven to 350°F and lightly butter a 13x9-inch baking dish.
Place yams and sweet potatoes in a large pot fitted with a steamer basket and steam for 15 minutes or until almost fully cooked. (Or place in a large covered bowl with 1 inch of water and microwave on HIGH.)
Place half in the bottom of prepared dish and top with half the pears. Repeat layers.
Stir together brown sugar and cinnamon and sprinkle over the top. Drizzle with syrup and dot with butter. Top with walnuts and bake for 30 minutes.