Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the potatoes and then slice them into chips about as large as your index finger. Tip them into a bowl. Add 1 tbsp olive oil and some salt and pepper. Toss a few times to coat in the oil.
Spread the chips out on a baking tray. Slide into the oven on the top shelf and bake for 20 mins.
Peel and finely chop the onion. Tip into a bowl. Finely grate in the lime zest and squeeze in the juice from one half. Halve the avocado. Scoop out the stone with a teaspoon. Scoop the flesh out of the skins. Finely chop it. Add to the bowl and toss to coat in the lime juice.
Finely chop the chives and add most of them to the bowl, keeping a pinch back for garnishing. Add a pinch or two of cayenne to the salsa (it’s spicy, so use as little or as much as you like). Season with salt and pepper. Stir a few times to mix.
Brush a baking tray with ½ tbsp olive oil. Rub 1 tsp Cajun seasoning into each cod fillet. Place them on the baking tray.
The chips should have been cooking for 20 mins by now. Turn them over and return to the oven on the middle shelf. Put the cod in the oven on the top shelf. Bake for another 10-12 mins. The chips should be golden and the fish white and opaque. It will flake easily when pressed with a fork.
Serve the cod with the chips and the avocado salsa, garnished with the remaining chives, and wedges of leftover lime.