Heat your oven to 180°C/Fan 160°C/Gas 4. Peel and finely chop the onion and carrot. Trim the dry ends off the celery and finely chop it to match the onion and carrot.
Warm a medium pan over a medium heat for 1 min. Add ½ tbsp oil and the veg. Season with a pinch of salt and pepper. Cook and stir for 5 mins till the veg is starting to look glossy.
While the veg fry, peel and crush the garlic. Finely grate the zest from the lemon (keep the lemon to one side for later). Stir the garlic and lemon zest into the veg. Drop in the bay leaf. Crumble in the stock cube. Stir in the barley and pour in 400ml boiling water. Cover, bring to the boil then turn the heat down and simmer for 15-20 mins till most of the water has been absorbed.
While the barley cooks, finely grate half the Parmesan onto a plate. Stir in half the breadcrumbs. Finely chop the parsley leaves and stir most of them into the breadcrumbs and cheese (keep a few back for garnishing).
Line a baking tray with baking paper and place the cod fillets on it, skin-side down. Spoon over the Parmesan breadcrumbs.
Place the cherry vine tomatoes on the baking tray alongside the cod. Slide into the oven and roast for 10-15 mins. The tomatoes will start to burst and the fish will be golden on top and flake easily when pressed with a fork.
While the fish bakes, whisk the juice of half the lemon with ½ tbsp olive oil and a pinch of salt and pepper to make a dressing. Toss the watercress with the lemon dressing.
Taste the barley and add a pinch more salt or pepper if you think it needs it. Drain off any excess water and spoon the barley onto 2 warm plates. Nestle a cod fillet next to the barley and scoop the tomatoes and watercress onto the plate. Garnish with the last of the parsley and serve with the remaining lemon, sliced into wedges.