Pre-heat the oven to 190°C.
Line a 7 inch straight-sided tart tin with greaseproof paper. Roll out the pastry on a lightly floured surface to 1cm thick and cut a large round of pastry slightly larger than the tart tin. Line the tin using the pastry, pressing in well and pricking the base. Trim any overhanging pastry and chill for 10 minutes.
Line the pastry case with baking parchment and fill with baking beans. Bake in the oven on a baking sheet for 15 minutes until golden at the edges. Remove the parchment and beans at this point, brush the base with the beaten egg and return to the oven to allow the base to cook and brown for 5 minutes.
Remove from the oven and cool on a wire rack. Combine the flaked tuna, chopped parsley and seasoning in a mixing bowl. Spoon into the base of the cooked pastry.
Beat together the eggs, milk, crème fraiche and seasoning until smooth. Pour on top of the tuna mixture and sprinkle the Gruyere on top.
Reduce the oven to 160°C.
Bake for 40-45 minutes until the quiche is set. Remove and garnish with coriander leaves before cutting and serving.