Warm the milk but do not boil in a saucepan. Add the bay leaf and infuse the milk; leave for a while. Make a white sauce: mix melted butter with flour to make a paste, gradually add the milk (add a little more milk if it's too thick). Add some salt and plenty of pepper, chopped parsley, garlic puree. Leave aside to cool. Remove bay leaf.
Roll out the puff pastry into a large square or circle whatever you prefer, remembering not to roll too thin or leave to thick - judge by the portion size you would like.
Open the tuna, drain and add to white sauce mix, mix gently, trying to keep some of the tuna chunks intact. Pour onto middle of puff pastry, pull pastry over and crimp edges with a fork or fingers. Brush with egg.
Bake in the centre of the preheated oven for 25 to 35 minutes, depending on size.
Add sweetcorn and onion for a different taste and texture.