Heat butter in a pot, put all vegetables and stir fry for 5 minutes on medium heat.

Add the water, make it to a boil and simmer for 15 minutes.

Purée #3 in a blender, put it back to the pot, add the milk, and simmer on low heat until it gets warm.

Place the miso paste into a ladle, add a little soup and whisk until smooth. Then add to the soup and stir. Salt to taste. Ladle into bowls. Top with croutons if desired.