Preheat oven to 200C (180C fan forced) and line a large baking tray with baking paper. Place tuna, corn, red onion, cheese, mayonnaise, parsley and salt and pepper in a bowl. Stir until throughly combined. Taste and adjust with salt and pepper if required.
Cut each pastry sheet in half and place two of the halves onto the prepared tray. Spread each half with the tuna mixture leaving a 2cm boarder all round. Using a little egg wash, brush around the edges.
With remaining pastry sheets, cut slits, at 1.5cm intervals along the short length of the pastry from one side to the other, but making sure you leave a 2cm boarder, uncut, on each of the 4 sides. Carefully lift each cut sheet and place on top of the tuna and gently stretch over the top and to the sides to cover. Crimp the edges with a fork to seal.
Brush top of pastry with remaining egg wash and sprinkle with sesame seeds, if using. Bake in the oven for 30-35 minutes or until golden brown. To serve, cut in half or into slices and serve with salad.