Trim the leeks and rinse out any grit. Finely slice them. Peel and thinly slice the onion. Pull the leaves from the cauliflower and discard any tatty ones. Finely shred the leaves. Set the cauliflower aside. Fill and boil the kettle.

Place a large, deep frying pan or casserole dish on a medium heat and add 100g butter. Melt the butter, then add the leeks, onion and cauliflower leaves to the pan. Cook for 15 mins, stirring regularly till the veg is soft. If the veg are ready while you're still preparing the rest of the pie, turn the heat off and set the pan aside.

While the veg are cooking, begin to make your white sauce. Pour in 750ml milk (save the rest for your Yorkshire puddings) into a large pan and drop in the thyme sprigs and 2 bay leaves. Add a good pinch of salt and pepper, then place the pan on a medium heat and bring till it just starts to bubble. Don’t let the milk boil, however. Let it simmer for 10 mins, then turn off the heat, popping a lid on the pan to keep warm.

Preheat your oven to 200°C/Fan 180°C/Gas 6. Chop the cauliflower into large florets. Remove the truffle cheddars from the wax wrapping and coarsely grate.

Pour the thyme and bay-infused milk through a sieve into a jug. Discard the bay and thyme.

Sprinkle the cornflour into the pan with the cooked leeks and stir well for 1 min.

Slowly pour the milk into the pan with the leeks, stirring well as you add it, till well combined. Dollop in the Dijon and wholegrain mustards. Bring back up to a gentle bubble, then add the cauliflower florets and cook gently for 5 mins, stirring once or twice.

While the cauliflower cooks, roll out the pastry. Remove the pastry from the fridge 5-10 mins before you want to roll it out. Lightly dust your work surface with a little flour. Gently knead the pastry into a ball, then roll it into a large circle or rectangle that will just cover your large chosen pie dish.

When the cauliflower has cooked for 5 mins, add all but a small handful of the grated truffle cheddar to the pan. Gently stir. Taste the sauce and add more salt or pepper, if you think it needs it. Carefully transfer your cauliflower cheese mix into a large oven-proof dish.

Place your pastry on top of the cauliflower cheese to cover it. Fold any hanging edges in to create a crust. Crack the egg into a bowl and beat. Brush the pastry with the beaten egg, then sprinkle over the remaining truffle cheddar.

Slide the pie into the oven and bake for 30 mins till the pastry is golden and the cheesy filling is bubbling around the edges.

Remove the pie from the oven and leave to stand for 10 mins before serving.