Boil cauliflower in hot water for about 5 minutes (to remove unpleasant smell). Drain.
Add 2 cups chicken stock and thyme and cook until cauliflower is soft, about 15 minutes.
Remove from heat and, using a hand held immersion blender, puree the soup (you can also use blender). Add milk and lemon juice. Season with salt and pepper. Boil for one more minute.