In a frying pan, cook leeks, garlic and potatoes in 1/2 of the butter and the olive oil over medium low heat, stirring often. Do not allow potatoes to brown.
Place mustard, salt, pepper, celery salt and flour together in a bowl. Gradually whisk in water and chicken stock until well blended. Stir into potato mixture and bring to the boil. Reduce to a simmer and cook for 1 hour.
Mash softened potatoes by hand so they remain lumpy. Stir in Cheddar and Parmesan cheeses until melted, then add milk but do not boil.
Cook mushrooms over medium high heat in remaining butter until soft. Add to the soup and stir. Serve immediately with croutons, if desired.
I stuck as closely as I could to the recipe. I used ordinary mushrooms that I had in. I thought it was just fine even before adding the mushrooms. It was tasty and thick. I would like to know if it is ok to freeze it. I'm intending to. - 19 Dec 2013