Heat a medium frying pan over medium-high heat. Cook prosciutto for 2 minutes, turning, or until browned and crisp. Transfer to a plate, allow to cool then crumble.
In the same pan, heat oil over medium-high heat. Add sage and cook for 1 minute or until light golden. Transfer to paper towel-lined plate and set aside.
In the same pan, add onion and cook for 5-7 minutes, stirring, or until softened and light golden. Add sugar and balsamic, and cook for a further 3 minutes or until thick. Remove from heat and set aside to cool.
Line 2 baking trays with baking paper. Place 2 pastry sheets on trays. Slice brie rounds in half horizontally to make four rounds. Place bottom half of each round on the centre of pastry sheets. Top with onion mixture, prosciutto and sage. Top with remaining brie halves, cut-side down.
Brush pastry edges with a little egg. Place remaining pastry sheets over filling, shaping to cover, and press onto pastry base. Trim pastry, leaving a 3cm border, and press edges well to seal. Brush all over with egg and sprinkle over a pinch of salt flakes. Refrigerate for at least 1 hour.
Preheat oven to 220°C. Bake pies for 25-30 minutes or until golden and puffed. Serve with tomato chutney.