In a medium bowl, stir together dates, bacon, chutney, walnuts and rosemary. Finely chop or grind remaining 1/4 cup walnuts and place in a smaal dish.
Place pastry on a board. Mound date mixture in center and spread level to 4" circle. Place Brie on top. Brush exposed pastry with water.
Gather and pleat pastry to cover Brie. Snip off excess dough with kitchen shears. Press with hands to flatten and seal sems. Invert onto parchment-lined baking sheet.
Preheat oven to 400°F. Bake for 20 minutes; brush with egg and bake 5 minutes more or until golden brown. Set aside to cool slightly, about 30 mintues. Transfer to platter and garnish with rosemary sprigs. Use a serrated knife ro cut into wedges. Make ahead: Recipe can be assembled and refrigerated, wrapped airtight, one day in advance up to and including Step 3. Or, double the recipe and freeze one for another occasion.TIP: Cut out decorative leaves with pastry scraps to decorate top of pastry.