For the pastry, put the flour in a food processor with the butter, mustard and thyme leaves, and pulse until the mixture resembles breadcrumbs. Add 4-6 tbsp cold water and pulse again until it forms a ball. Wrap and chill for 30 mins.

For the filling, break the cauliflower into small bite-sized florets (keep the leaves for stock or roast until crisp and serve alongside the pie), and chop the stalk into 2cm pieces. Steam in a steamer or steamer basket set over a pan of simmering water for 8-10 mins until the stalk pieces are just tender, but not cooked through. Remove from the steamer and leave to cool and steam-dry on a plate.

Heat the oil in a frying pan over a low heat and fry the leeks with a pinch of salt for 10 mins until softened. Add a splash of water if they start to turn lightly golden and cook until the water has evaporated. Remove from the heat.

Cut off a third of the pastry and set aside. Roll out the rest on a floured work surface to a round large enough to line a 25cm wide and 7cm deep pie dish (enamel pie dishes work well for this). Line the dish and set aside.

Whisk the eggs in a large bowl with some seasoning. Reserve a little of the beaten egg in a small bowl, then fold the cheese, cauliflower and leeks into the remaining egg. Tip the filling into the pie dish and spread out evenly. Roll out the remaining pastry and use to cover the pie, using some of the reserved egg to seal it to the edge. Crimp the edge and trim any excess, then chill for 20 mins.

Heat the oven to 200C/180C fan/gas 6. Brush the top of the pie with the remaining beaten egg and bake for 45 mins until golden and crisp on top. Stand for 10 minutes before serving with roast potatoes.