Heat the oil in a casserole over medium heat and cook and stir the onion, garlic and ginger until the onion is tender, about 5 minutes. Stir in the curry powder and cauliflower.
Pour in the tomatoes and water and bring to the boil. Reduce to a simmer, cover and cook until the cauliflower is tender, about 15 minutes.
Add peas and coriander, and cook until the peas are hot, about 3 minutes. Stir the yoghurt and flour together in a small bowl. Stir the yoghurt mixture and the prawns into the sauce and cook over low heat, stirring, for about 2 minutes or until slightly thickened. Serve hot.