Preheat oven to 180ºC. Line an oven tray with baking paper.
Remove puff pastry sheets from packaging and set out to defrost.
While pastry is defrosting, prepare filling. Combine tuna, cream cheese, red onion, lemon juice, salt and pepper in a large bowl and mix well.
Using a cookie cutter, cut out six pastry circles per sheet of pastry.
Add roughly one tablespoon of tuna cream cheese mixture to the centre of each pastry round. Press pastry together into a semi-circle shape and seal edges using a fork to crimp. repeat with remaining pastry and filling.
Lay puffs on to prepare baking trays. Brush each with egg wash and sprinkle with sesame seeds.
Bake for 20-25 minutes, until puffs are golden and crunchy. Remove from the oven and allow to cool slightly before serving. Tuna puffs can be served warm or at room temperature.