Heat olive oil in a pot over medium heat and cook onion until soft and translucent, about 5 minutes. Add potatoes and water and bring to a boil. Cover and simmer until potatoes are almost soft, about 10 minutes.
Stir cauliflower, coconut milk, and turmeric into the pot, cover, and cook at a gentle simmer over medium heat until cauliflower is soft, about 10 minutes. Remove lid, season with salt and pepper, and sprinkle with cilantro.