Heat the oven to 200C/180C fan/gas 6 with a baking tray inside. Melt the butter in a small pan over a medium heat and, once foaming, fry the onion with a large pinch of salt for 10-15 mins until softened and beginning to caramelise. Remove from the heat and set aside to cool slightly. Whisk the eggs and milk together, and season well.

Butter an 18 x 25cm baking dish that's at least 6cm deep. Line with baking parchment. Roll the pastry out into the tin, making sure the edges reach the corners. Scatter over the caramelised onion and grated cheese, then carefully pour in the egg mixture. Transfer the dish to the hot baking tray in the oven, and bake for 35-40 mins until golden and puffed up.