Preheat oven to 375F. Lightly spray an 8-in. square baking dish. Set aside. Peel pears and slice in half lengthwise. Scoop out core using a 1-tsp measuring spoon. Arrange pears in one layer, cut-side up, in prepared dish. Split vanilla bean in half lengthwise. Use the dull side of a paring knife to scrape out seeds. Combine seeds with brown sugar and butter in a small microwave safe bowl. Microwave for 30 sec until butter is melted. Stir in lemon juice. Generously brush vanilla mixture over pears, then pour rest of mixture into baking dish.
Roast in centre of oven until pears are tender, about 40 min. Serve with whipped cream.