Pre-heat your oven to 180C Fan/200C/400F/Gas Mark 6. Take the pastry out of the fridge, un-package it but don't unroll it yet
Warm the oil and butter in a pan on a low heat then add the onion, garlic and leeks. Cook them on a low heat until they are soft, then set them aside
Mix the cubes of squash in a bowl with a drizzle of vegetable oil and the sage until they are fully coated
Roll the pastry out onto a baking tray, if it doesn't come with baking paper make sure the tray is lined with it
Pour the leek mixture onto the pastry, leaving a 1-2cm border all around the edges
Add the butternut squash, and crumble the stilton all over
Brush the milk in the empty border of the pastry all around the edges
Bake the tart for 25 minutes until it is golden and bubbling
Serve hot, or allow to cool, refrigerate and re-heat as desired