Tip the pearl barley into a small pan. Pour in enough boiling water to cover it by a couple of cms. Add a pinch of salt, pop on a lid and bring to the boil. Turn the heat down to low and simmer for 20-25 mins till the barley is tender.
While the barley simmers, finely shred the spring greens, discarding any really thick bits of stalk. Peel and dice the onion. Dice the courgette. Peel and finely chop the garlic. Halve the chilli. Flick out the seeds and white bits for less heat. Finely chop the chilli. Roughly chop the parsley leaves.
Warm 1 tbsp olive oil in a deep frying pan or wok. Add the onion and courgette. Season. Stir and fry for 5 mins to soften the veg.
Add the garlic and chilli to the veg. Stir and fry for 1 min till the pan smells aromatic. Add the tin of chopped tomatoes. Quarter-fill the can (around 100ml) with water and pour that in too.
Cover the pan with the lid. Bring to the boil. Turn the heat down and simmer for 5 mins till the sauce starts to thicken.
Slide the cod into the pan, skin-side down. Cover. Simmer for another 5 mins till the sauce is thick and the cod is opaque and cooked through.
While the cod cooks, pop the shredded spring greens into a pan. Cover with boiling water. Bring to the boil, then lower the heat and simmer for 2 mins till it's just cooked. Drain.
Drain the pearl barley. Taste the arrabiata sauce and season as needed. Serve the cod and arrabiata sauce with the spring greens, sprinkled with the chopped parsley leaves.