Dice the onion into small cubes. Heat 1 tablespoon olive oil in a pan, add the onion, and cook on medium heat until golden.
Drain the tuna and add it to the pan. Season with salt, pepper, curry, mustard, parsley, and lemon juice. Stir and cook for about 5 minutes, just until the liquid is gone and the flavors blend.
Turn off the heat. Stir in the grated parmesan, then let the mixture cool completely.
Roll out the puff pastry and spread the filling evenly. Press it down gently, then roll it up. Wrap tightly in baking paper or cling film and press like a sushi roll.
Place in the fridge and let it chill for about 30 minutes. You need to bake the puff pastry while it's cold.
Unwrap the roll. Brush with milk or egg wash, sprinkle sesame seeds if using, and press them down so they stick. Slice into about 16 pieces. Place on a baking sheet and bake at 200°C (400°F) for 20–30 minutes, until puffed and golden.