remove the base and dark green tops of the leek; you will only want to use the light green and white parts. halve lengthways, then chop into half moons roughly half a cm in thickness. 

warm a generous amount of olive oil in a heavy based stock pot. add the leek and toss to coat in oil and allow to cook until soft and translucent. add the butter while the leek is cooking, if using. 

add the mustard and stir as it releases it's flavour, approx 30 seconds 

add all of the cauliflower and stir to coat in mustard

add all of the stock and bring to the boil, then reduce to a low heat

take your piece of gruyere and cut off the rind (the natural skin of the cheese), leaving it in one piece if possible. if there is any paper attached to the rind, remove and discard. add the rind to the stock to add the natural earthy flavour of the cheese to the stock. grate the cheese and reserve.  

leave to cook for about an hour on low heat to allow the flavours to develop. remove the gruyere rind before blending - this is key or you will end up with a grainy texture. 

add the cream and about half of the grated cheese and blend to a smooth consistency. if it's too thick, you can add a little bit of milk or water to thin it out a little, you may need to adjust the seasoning accordingly. season with salt and pepper. 

to serve, ladle into warm bowls and dress with a drizzle of oil (see note) and cracked black pepper. 

for cheesy toasts, toast or pan-fry slices of sourdough, spread with a lavish amount of butter then top with the remaining gruyere cheese. pop under the grill for a few minutes to melt the cheese, crack some pepper over the top and serve immediately.