Preheat the oven to 200C/400F/Gas 6.
Roll out the pastry on a lightly floured surface to make a 20cm/8in round and place onto a non-stick baking sheet.
Heat a frying pan over a low-medium heat and place the spinach into the dry pan. Stir over the heat until wilted, then sprinkle over the pastry.
Dot the cream cheese over the spinach and season, to taste, with salt and freshly ground black pepper.
Bake in the oven for 10-12 minutes, or until golden-brown and crisp.
Heat the oil in a small frying pan, add the onion and fry gently for five minutes.
Add the salmon pieces and continue cooking for 2-3 minutes, or until the salmon is cooked through.
To serve, place the puff pastry pizza onto a serving plate and scatter the salmon and onion over the top.