Drop the button mushrooms into a very hot pan and cook until all the liquid has evaporated and they are dry.

Once dry, add the butter and cook until the mushrooms are caramelised and golden brown.

Drain off any excess butter and just barely cover the mushrooms with water. Bring to the boil.

Remove the mushrooms from the heat and pour into a container. Seal with a lid and put in the fridge overnight to infuse.

The next day, strain the liquid through a fine sieve and into a saucepan.

Heat the liquid over a high heat until reduced, dark and syrupy.

Add the cream, bring to a simmer and cook one minute more.

Remove from the heat and adjust the seasoning to taste with salt and lemon juice.

Bring a large pot of salted water to the boil.

Cut out the potato slices into rings using the 6cm pastry ring. Add the potatoes and cook until tender.

Meanwhile, melt 25g butter and add the spinach. Cook briefly, until just beginning to wilt, then tip into a colander to drain.

Put the spinach into a clean tea towel and squeeze out all the water. Season the spinach heavily with salt and pepper, then put in the fridge to chill.

Add the remaining 100g butter to a separate pan, and melt over a medium-high heat.

Add the portobello mushrooms, skin side down, and cook on a medium heat until golden brown.

Season with salt and flip the mushrooms. Add the chopped garlic and thyme leaves.

Cook the mushrooms on the other side, basting with the garlicky butter, for 5 minutes more then season again and remove from the heat. Allow the mushrooms to rest for 10 minutes.

Take the mushrooms out of the pan and pat them dry.

Cut out 8 discs from the centre of the mushrooms using the 6cm pastry ring. Chill the cut mushrooms in the fridge.

Place a 6cm pastry ring on a square of baking parchment. Put a mushroom into the bottom of the ring and then top it with a round of cheese.

Press in a thin layer of dry spinach and then another disc of cheese. Add a potato disc, then more cheese, spinach again and more cheese.

Finish with a mushroom disc (facing skin side up).

The final layers should be: mushroom, cheese, spinach, cheese, potato, cheese, spinach, cheese, mushroom. If the stack is too high you may need to trim the mushrooms horizontally. Press down tightly.

Repeat this process 3 times to make 4 stacks, and then put them in the fridge (still in their pastry rings) to set.

Place four of the 120mm pastry discs onto squares of baking parchment.

Put one ring of filling on each pastry disc, then remove the ring.

Brush egg yolk around the edge of the stack of filling.

Place another 120mm pastry disc over the top of the filling and then smooth the pastry down the sides of the stacks, squeezing out the air as you go and closing it at the bottom.

Crimp the base to seal the two pieces of pastry and brush egg yolk all over.

Put the pithivier in the fridge to set the egg wash, about an hour. Repeat this egg washing process two more times, leaving the pastries to dry each time, so you have 3 layers of yolk.

While the egg wash dries, make the leeks. Add the butter to a medium frying pan over a medium-high heat.

Add the leeks, thyme and garlic to the pan.

Cook the leeks, without stirring, until they are golden brown on one side. Season with salt.

Turn the leeks over, and brown again on the other side. Taste and adjust the seasoning, then pull the pan off heat and leave the leeks to cool.

Once you have finished egg washing the pastries, cut a small hole in the top of the pastry to release steam as it cooks, and cut off the excess pastry from the base leaving a small lip. You’ll need a cutter roughly 7cm in diameter.

Preheat the oven to 180°C/gas mark 4.

Score the domes of the pithiviers for decoration and put them on an oven tray.

Bake for 20-25 minutes, or until golden brown.

Warm through the leeks and the mushroom bisque. Divide the leeks between 4 serving bowls.

Place a pithivier on each stack of leeks and pour around the warm mushroom bisque.