In a skillet, cook leek, garlic, and potato in 1 tablespoon of butter and the olive oil over medium low heat, stirring often. Do not allow potato to brown.
Place mustard, salt, pepper, celery salt, and flour together in a bowl. Gradually whisk in water and chicken broth until well blended. Stir into potato mixture, and bring to a 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 1 tablespoon of butter until soft. Add to the soup, and stir. Serve immediately with croutons, if desired.
