Check over the fish for any stray bones, then cut into chunky pieces.
Heat 2 tablespoons of the oil in a large flameproof casserole or saucepan and gently cook the mustard and fenugreek seeds until they start to pop. Add the coriander, chilli and onion and cook very gently, stirring, for 5 minutes.
Stir the garlic, ginger and turmeric into the pan, then spoon the ingredients out on to a plate. Add the remaining oil to the pan and fry the fish for 3–4 minutes, until coloured on all sides, turning the pieces gently. Lift out with a slotted spoon on to a separate plate.
Return the spice mixture to the pan and add the coconut milk. Bring to a gentle simmer, cover and cook for 10 minutes.
Stir in the tomatoes and return the fish to the pan with the squid. Cover and cook gently for a further 10 minutes before serving in shallow bowls.