Heat the oil in a medium saucepan over medium heat. Add the leeks and cook, stirring, until tender, 3 to 4 minutes. Add the potato, ginger, rosemary, 1/2 teaspoon salt and a few grinds of pepper and cook, stirring, until the potatoes are well coated, 2 to 3 minutes. Pour in the broth and 1 1/2 cups water and bring to a simmer.
Add the cauliflower and garlic and simmer until the potatoes, cauliflower and garlic are tender, about 20 minutes, adjusting the heat as needed to maintain a simmer. Remove from the heat, and let cool at least 5 minutes. Remove and discard the rosemary and carefully puree until smooth with a handheld immersion blender or in 2 to 3 batches in a blender (do not fill blender more than halfway full).
Add the cream and reheat the soup if necessary. Adjust the consistency with water and seasoning with salt and pepper.
Serve hot sprinkled with the parsley and a few grinds of pepper.
When blending hot liquid, first let it cool for 5 minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters and pulse until smooth.
Copyright 2016 Television Food Network, G.P. All rights reserved.