Melt the butter in a large pot over a low heat.
Add the leek and saute for 5 minutes, or until softened.
Add the cauliflower, potato, chicken stock, milk and bay leaves to the pot.
Season everything well with salt and pepper.
Bring to the soup to the boil. Take care here if you are using a small pot, as the added milk can cause the pot to boil over.
Reduce the heat to low, and cover the pot with a lid.
Simmer the soup for 30 minutes, or until the potato and cauliflower are cooked through and are soft.
Remove the pot from the heat.
Remove the bay leaves from the soup. Do not skip this step or you will end up with tough bits of bay leaf through your soup.
Blitz the soup until smooth. I use a hand held blender to do this.
Return the pot to a low heat and stir in the grated cheddar, (and mustard if using).
Heat for about one minute, stirring constantly, until the cheese is melted.
Serve with crusty bread and butter