Blueberry or Raspberry Pastry « Trottoir »

Ingredients

• • 1/2 cup (125 ml) sugar
  • 3 tablespoons (45 ml) cornstarch
  • 6 cups (1.5 litres) blueberries or raspberries, fresh or frozen and thawed (see note)
  • 1 tablespoon (15 ml) lemon juice
  • 1 piece 1 1/2 lb (675 g) puff pastry, thawed (preferably all butter)
  • 1 egg
  • 1 tablespoon (15 ml) water
  • Coarse sugar