1. For the filling: Preheat the oven to 400 degrees F and position the rack in the center of the oven. Coat a 13-by-9-by-2-inch baking dish with 1 tablespoon of butter.
2. In a large bowl, combine the sugar, flour and cinnamon. Scrape in the seeds from the vanilla bean and whisk to blend. Add the nectarines and lemon juice and toss to coat. Set the mixture aside while preparing the biscuit topping, allowing the nectarines to release some of their juices, and tossing occasionally.
3. Meanwhile, prepare the biscuit topping: In a large bowl, whisk the flour, 1/4 cup of the sugar, baking powder and salt. Using your fingertips, rub in the butter until the mixture resembles a coarse meal. In a small bowl, mix the egg and 3/4 cup of the cream to blend. Using a wooden spoon, gently stir the egg mixture into the flour mixture just until the dough forms (do not overmix the dough).
4. Transfer the nectarine mixture to the prepared baking dish, arranging it in an even layer. Using about 1/4 cup of dough for each biscuit, gather the dough and gently form it into discs. Arrange the dough discs over the fruit, spacing them evenly apart. Brush with the remaining 2 tablespoons of cream and sprinkle the remaining 2 tablespoons of sugar over.
5. Bake uncovered until the biscuit topping puffs and becomes golden brown and the juices are bubbling, about 50 minutes. If the biscuits brown nicely before the filling bubbles, lay a sheet of foil over the biscuits. Cool on a rack for at least 15 minutes. Serve warm with vanilla ice cream.