In a large saucepan, heat the olive oil over medium heat. Add the onion, ginger and 1 teaspoon salt and cook, stirring occasionally, until softened, about 5 minutes. Add the curry powder and cumin and cook, stirring, for 3 minutes. Stir in the cider, scraping the bottom of the pan. Add 4 cups water and the sweet potato and bring to a boil. Add the cauliflower, lower the heat and simmer until the vegetables are tender, about 10 minutes.
Using a blender, and working in batches, puree the soup; return to the saucepan. Stir in the chicken and warm through over medium heat; season with salt and pepper.