Peel and slice the spring onions, garlic and ginger. Cut the fish into chunks. Heat the olive oil and stir-fry the mustard seeds and curry leaves until the seeds start to pop. Add the spring onions, garlic, ginger and chilli and cook for 5 minutes. Mix the turmeric with 1 Tbsp water and then stir into the pot. Fry for 1 minute, then add the fish, coconut milk and tomatoes. Bring to the boil, then simmer for 20 minutes. Serve with chopped coriander and cooked quinoa or brown basmati rice.