Preheat oven to 400°F. Place squash, skin side down, on a rimmed baking sheet lined with foil. Drizzle with oil; season with salt and pepper to taste. Roast until tender, about 30 minutes.
In a small saucepan, melt butter and maple syrup. Turn oven to broil. Brush squashes with butter mixture and place a few walnuts on top. Place under broiler until deeply browned, about 2 minutes.