Place cauliflower in a pot with stock, milk, and salt and bring to a boil.
Cook until stalks can be easily pierced with a knife, but are still firm (7 minutes).
In a small saucepan, make a roux, by heating the butter and mixing in the garlic and flour, stirring constantly until flour turns golden beige (3 minutes) (be careful not to scorch the flour and not to get any on you).
Add the parsley and white pepper.
Thin the roux with 1/2 cup of the cauliflower broth and slowly pour back into the soup.
Simmer until cauliflower is done and soup no longer tastes floury (10 minutes).
Let cool thoroughly.
In a small bowl stir a little of the soup into the sour cream until smooth; return this to the remaining soup.