Preheat oven to 375° F.
Place the sugar in a small bowl. With a sharp knife, split the vanilla bean in half lengthwise and scrape the seeds into the sugar. Reserve the pod.
Peel the pears, cut them in half lengthwise, and remove the cores. Arrange the pears in a baking dish cut-side up. Drizzle them with the lemon juice and sprinkle on the vanilla sugar and butter. Add the vanilla pod and 2 tablespoons of water to the dish. Roast the pears for 30 minutes, spooning the pan juices over the tops occasionally. Turn the pears and continue to roast, basting once or twice, until tender, 25 to 30 minutes more. Divide the pear halves among 8 serving dishes. Serve with a dollop of crème fraîche or a scoop of ice cream, if desired.
Tip: You can use any variety of pear, but the slender Bosc variety has a sweet flavor and an elegant appearance that work like a charm in this recipe.