Preheat the oven to 180℃ fan.

Begin with the pastry. Blitz the flour, cubed butter and a pinch of salt in a food processor until the mixture resembles breadcrumbs. Or rubber together with your hands. Add the Greek style yogurt, and pulse until the mixture comes together to form a dough. Or use one hand to bring the dough together. Now shape into a disc, pop into a freezer bag and chill for 30 minutes.

Soak the sliced leeks in cold water to remove any soil or grit, then drain and dry with a clean tea towel. Melt the butter in a shallow pan over a low-medium heat and cook the fennel and leeks with the sage leaves and a generous seasoning of salt and pepper for 15 min, until really soft and sweet. Set aside to cool.

Beat the egg and egg yolk in a bowl. Add the sour cream, wholegrain mustard and add two thirds of the cheese to the bowl. Stir in the cooled fennel and leeks and season to taste. on a lightly floured work surface roll the pastry into as large a disc, then place on a sheet of baking paper. Lift the baking sheet carefully onto a baking tray.

Spread the creamy vegetable mixture evenly on top, leaving a 1-2cm gap around the edge. add the sage leaves, then fold the edges over, overlapping the pastry to create a few creases. Sprinkle with the remaining cheese. Brush edges with beaten egg.

Bake for 40 to 45 minutes, or until the pastry is golden brown. Allow to cool for 10 minutes on the baking sheet. Transfer to a plate and serve with a salad.