In a large soup pot or dutch oven, melt 4 tablespoons butter. Add the onion and cook for a few minutes, or until it starts to turn brown.

Add the carrots and celery and cook an additional couple of minutes. Add cauliflower and parsley and stir to combine.

Cover and cook over very low heat for 15 minutes. After 15 minutes, pour in chicken stock or broth. Bring to a boil, then reduce heat and allow to simmer.

In a medium saucepan, melt 4 tablespoons butter. Mix the flour with the milk and whisk to combine. Add flour-milk mixture slowly to the butter, whisking constantly.

Remove from heat and stir in 1 cup half-and-half. Add mixture to the simmering soup. Allow to simmer for 15 to 20 minutes. Check seasoning and add more salt or pepper if necessary.

Just before serving, place the sour cream in a serving bowl or soup tureen. Add two to three ladles of hot soup into the tureen and stir to combine with the sour cream. Pour in remaining soup and stir.

Serve immediately.