Grind the coriander seeds to a fine powder. Mix together well with a pinch of salt and the lime zest, then pour out on to a flat baking tray. Cut the tuna loin lengthwise into quarters so you have 4 fillets. Roll the tuna in the coriander mix. Lightly smear with 1 tsp of vegetable oil and place over a high heat until the pan is smoking. Add the tuna fillets to the hot pan one at a time and cook until all the sides are sealed and blackened.
Place the fillets on a tray and put them in the fridge. Put the ginger, chillies and garlic and mash to a pulp. Once pulped add the juice from the limes and pour everything into a mixing bowl together with the vegetable oil and mix well, stir in the soy sauce and coriander.
Remove the tuna from the fridge and cut each fillet into thin slices. Lay the slices of tuna onto the plate and spoon over the dressing.