Personally I think I'd do this

steam the spinach for a minute
poach the fish in a bit of milk then use the milk to make a cheese sauce
mix all together (after squishing spinach)
roll out the filo, score a square a cm from the edge all the way around and then plop your mixture on. Cook until the pastry is puffed up and golden/

thanks - good tip re the spinach

SOunds good but squish the water out of the spinach before putting on the pastry...

what do you think - if i steam the spinach, the put it on the fish which i then wrap in filo?

nice?

how long should i cook it for?