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

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

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

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

Remove from heat and stir in the half-and-half. Add this mixture to the simmering soup, along with 1 to 2 bay leaves. Allow to simmer for 15 to 20 minutes, then taste for seasonings and add more salt or pepper if desired.

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