Pre-heat the oven to gas mark 6, 200ºC. First, pop a baking sheet into the centre of the oven while it’s pre-heating.
Meanwhile, melt the butter in a frying-pan, add the chopped spring onions and leeks and cook them over a medium heat for 5-6 minutes.
Now, using oven gloves, place the pastry case, still in its foil container, on the baking sheet, arrange the onion-and-leek mixture over the base and crumble the Roquefort on top.
Whisk the double cream and the eggs together in a jug, season and then pour this over the cheese and leeks.
Sprinkle the Parmesan over the surface, then transfer the baking sheet to the centre of the oven. Reduce the heat to gas mark 4, 180ºC and bake the tart for 35-40 minutes, till golden and firm in the centre.
Leave to rest for 10 minutes before serving with a tossed green salad.
Note: this can also be made in 6 Jus-Rol frozen tartlet cases; cook as above.