Put the haddock in a pan, cover with boiling water and simmer for 3 minutes. Remove the fish from the pan and flake. Pour boiling water over the spinach until wilted, then squeeze away all the excess water.
Mix together the crème fraîche, beaten eggs (reserving 1 tablespoon) and spring onions and season well. Mix half this mixture with the spinach. Unwrap the pastry on to a baking sheet. Score a 1 cm (1/2 inch) border around the edges with a sharp knife and brush the border with the reserved egg.
Spoon the spinach mixture into the centre of the tart and scatter the haddock on top. Spoon over the remaining crème fraîche mixture and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes or until golden and cooked through.