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Slice the tuna into 1cm strips, then dice into rough 1cm cubes. Drizzle over the olive oil and scatter over the lime zest, stir and put in the fridge for 20 mins or so. Gently toast the cumin and coriander in a small frying pan, then tip into a pestle and mortar with a pinch of salt and the chilli flakes and crush to a coarse powder. Stir the spice mix into the tuna with the lime juice and put the bowl back in the fridge for at least 10 mins or up to 1 hr.
To make the avocado purée, put all the ingredients in a food processor with a pinch of salt and blitz until you have a smooth purée, adding a little oil if it’s too thick to blitz. Spoon the mixture into a container.
If you like your tacos crispy, heat oven to 180C/160C fan/gas 4, brush them with a little oil, place on a baking sheet and cook for 10-15 mins. To build each taco, spoon on some avocado purée and spread out to the edge. Spoon on the spiced tuna and sprinkle over the chillies, pomegranate seeds, coriander and spring onions, drizzle with more oil and add a lime wedge to the plate. Eat with your hands, if soft, or a knife and fork if crispy.