Lay the pastry (a piece about 18cm x 23cm) on a lightly floured surface.
Cut into 4 rectangles (see Garys tip, below).
Place on a baking sheet and bake for about 15 mins until golden and puffy.
Meanwhile, pour the wine and 450ml/16fl oz water (see Garys tip below) into a large, deep pan and bring to a boil. Tip in the cleaned mussels and cover with a lid. After a couple of mins, remove the lid and stir the mussels until they open. Any refusing to open should be discarded.
Drain the mussels in a colander, saving the juices.
Remove the mussels from the shells, keeping some shells if you like, to use when serving the soup.
Warm the olive oil in the pan, then add the sliced onion, celery, star anise, orange zest, bay leaves, saffron and lemon juice. Cook over a moderate heat without colouring the vegetables, stirring occasionally, until completely tender, 12-15 mins.
Strain the mussel cooking liquor through a fine sieve over the vegetables, and bring to a simmer.
Add the tomatoes, season with a pinch of cayenne pepper or crushed chillies and simmer for a few mins to soften the tomatoes. Spoon the mussels and prawns into the soup, stir in your preferred quantity of butter and heat to just below simmering point you dont want to overcook the fish.
Sprinkle in some chopped parsley to finish, and serve in bowls with a pastry lid perched on top of each.