Meanwhile, heat the vegetable oil in a pan and cook the onion with some seasoning for 8-10 minutes or until soft. Add in the garlic, ginger and chilli, and cook for 2 minutes. Stir in the mustard seeds, turmeric, cumin and plain flour, and cook for 2-3 minutes. Gradually add the coconut milk, and some seasoning, stirring until a smooth sauce forms. Simmer for 4-5 minutes or until thickened. Stir in the cod, haddock, prawns, coriander, cherry tomatoes and lime juice, then tip out into a 20 x 30cm baking dish.