Cut the pastry into 4 pieces, then roll each piece out on a lightly floured surface until large enough to line the base and sides of a buttered 12 cm (5 inch) fluted loose-bottomed tart tin. Lift the pastry into the tins, press over the base and sides, then trim the top of the pastry with scissors so its stands a little above the top of the tins. Put on a baking sheet and chill for 15 minutes while making the filling.
Put the salmon in the top of a steamer, cover and cook for 8–10 minutes until the salmon can be broken into flakes easily and the flakes are pale pink all the way through. Lift out and put on to a plate, remove the skin (if there is any) and break into flakes, checking for and removing any bones.
Heat the butter in a small frying pan, add the spring onions and courgette and fry gently for 3–4 minutes until softened. Beat the eggs, milk and tarragon together with a little salt and pepper, then stir in the cheese.
Pour the egg mixture into the tarts, add the flaked fish, onion and courgette mix and bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 20–25 minutes until golden and the filling is just set. Serve warm or cold.