In a large soup pot or Dutch oven, melt 4 tablespoons butter over medium-high heat. Add the onion and cook until it starts to turn brown, 4 to 5 minutes.

Add the carrots and celery and cook about 2 minutes more. Add the cauliflower and parsley and stir to combine. Cover and cook over low heat for 15 minutes.

Pour in the chicken stock or broth. Bring to a boil, then reduce the heat to medium-low and allow to simmer for about 10 minutes.

In a medium saucepan, melt the remaining 4 tablespoons butter over medium-low heat. In a medium bowl, whisk to combine the flour and milk. Slowly add the mixture to the butter, whisking constantly. Let it cook until thickened, about 5 minutes.

Remove from the heat and stir in the half-and-half. Add the mixture to the simmering soup. Drop in a bay leaf or 2. Allow to simmer for 15 to 20 minutes. Taste for seasonings and add more salt or pepper, if you like.

Just before serving, place the sour cream in a serving bowl or soup tureen. Add 2 to 3 ladles of hot soup into the tureen and stir to combine with the sour cream. Pour in the remaining soup and stir. Or, serve in bowls and top each with sour cream. Serve immediately.