Heat your oven to 140°C/Fan 120°C/Gas 1. Peel and finely slice the red onion. Place in a bowl and finely grate in the lime zest. Squeeze in the lime juice and add a big pinch of salt. Use your hands to scrunch the onion and lime together for 1 min. Set aside to pickle while you make the fajitas.

Chop the cod fillets into 6-8 large chunks each, and pop them in a dish. Drizzle over 2 tsp olive oil and 1 tbsp Cajun seasoning. Rub the spices into the cod. Set aside for 5 mins to marinate.

Take the tortillas out of their packaging and wrap them loosely in foil or baking paper. Place in the oven to warm through. Pick any really thick stems out of the watercress, so you just have tender leaves and shoots.

Place a large frying pan on a medium-high heat for 2 mins. Add 1 tbsp olive oil and the cod chunks. Fry for 2-3 mins till golden underneath, then turn the pieces over and fry for another 2-3 mins. The cod should be golden brown and cooked through – it will flake easily when pressed with a fork. Take the pan off the heat.

Unwrap the tortillas and place on 2 warm plates. Top with handfuls of the watercress and the fried cod. Spoon over some lime-pickled onions and a little of the pickling liquid. Top with a spoonful of yogurt and serve.