In a large saute pan, cook the onions in the oil over medium heat until softened and slightly golden in color.
Add the cauliflower, potato, apple, curry and salt and stir-fry for a minute or two. Add enough water or vegetable stock to just cover the vegetables. Lower the heat slightly and cover the pan. Cook 10 -15 minutes, until the vegetables are tender when pierced.
Let the mixture cool slightly before blending to a smooth puree in a blender or food processor, adding more liquid if necessary depending on how thick you want your soup. Add the creme fraiche and taste for seasoning, adding more salt or pinch of sugar to taste.