Preheat the oven to 200C/400F/Gas 6.
Brush the filo pastry with butter, fold in half and press into a chefs' ring on a baking sheet. Brush with more butter, then transfer to the oven and bake for eight minutes, or until golden-brown.
Meanwhile, heat the olive oil in a small pan, add the leek and sauté for 3-4 minutes, until softened. Leave to cool slightly.
Place the eggs, cream and chives into a bowl, season with salt and freshly ground black pepper and mix well. Add the leeks to the bowl and stir to combine.
Remove the filo pastry case from the oven, pour in the egg mixture and top with the Stilton. Return to the oven and bake for 8-10 minutes, or until the filling has set.
To serve, place the tart onto a plate, remove the chefs' ring and sprinkle with chopped chives.