Preheat the oven to 220°C, fan 200°C, gas 6. Line a baking tray with a silicone mat or baking paper, put the cauliflower florets on the tray, drizzle over 1 tablespoon oil, season with sea salt and paprika and massage into the florets. Roast for 15 minutes.
Meanwhile, season the cod with salt, pepper and the harissa. Rub in with 1 tablespoon oil on both sides of the fish.
Once the cauliflower has been in the oven for 15 minutes, turn the oven down to 180°C, fan 160°C, gas 4. Move the florets to the edges of the tray, place the fish in the centre and return to the oven for 10-12 minutes.
To make the salsa, mix the tomatoes with the lime juice, chilli, 1 tablespoon oil and half the chives.
Serve the cauliflower topped with the fish and salsa. Scatter over the rest of the chives.
