Finely grate the zest from the mandarin and tip into a bowl. Squeeze in the juice. Add 1 tbsp caster sugar and 1 tsp sea salt. Stir to mix.
Peel and finely slice the red onion. Stir into the mandarin pickling brine. Leave to marinate for 30 mins.
Finely slice the lettuce into shreds. Halve the pepper. Scoop out the sides and white bits. Finely slice it.
Chop the fish into bite-size chunks. Pop in a bowl with 1 tsp cumin, a pinch of chilli powder and some salt and pepper. The chilli powder is hot, so use as much or as little as you like. Turn to coat in the spices.
Set your oven to its lowest temperature. Wrap the tortillas up in a square of foil and pop them in the oven to just warm through a little.
Warm a large frying pan for 1 min over a medium heat. Add 1 tbsp olive oil and the haddock. Fry for 5-6 mins, gently turning the fish now and then so it cooks through and is a little browned. When the flesh is opaque and flakes easily, it’s ready. Scoop into a heatproof dish and keep warm in the oven.
Add the pepper to the frying pan with ½ tbsp oil. Stir fry for 5 mins till softened and a little browned. Scoop into a separate bowl. Lay a warm tortilla on a plate and top with a little shredded lettuce, haddock, pepper and pickled onions with a drizzle of the brine. Repeat with the other tortillas. Serve straight away.