Step 1

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

Step 2

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

Step 3

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

Step 4

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

Step 5

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

Step 6

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

Step 7

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

Step 8

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

Step 9

Bring a large pot of salted water to the boil

Step 10

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

Step 11

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

Step 12

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

Step 13

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

Step 14

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

Step 15

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

Step 16

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

Step 17

Take the mushrooms out of the pan and pat them dry

Step 18

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

Step 19

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

Step 20

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

Step 21

Finish with a mushroom disc (facing skin side up)

Step 22

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

Step 23

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

Step 24

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

Step 25

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

Step 26

Brush egg yolk around the edge of the stack of filling

Step 27

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

Step 28

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

Step 29

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

Step 30

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

Step 31

Add the leeks, thyme and garlic to the pan

Step 32

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

Step 33

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

Step 34

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

Step 35

Preheat the oven to 180°C/gas mark 4

Step 36

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

Step 37

Bake for 20-25 minutes, or until golden brown

Step 38

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

Step 39

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