Heat your oven to 180°C/Fan 160°C/Gas 4. Peel and finely slice the shallots. Thickly slice the mushrooms. Peel and grate or crush the garlic. Finely chop the parsley stalks, keeping the leaves aside for later.
Pour 350ml boiling water into a small pan, pop on a high heat and bring back to the boil. Rinse the barley under cold water. When the water is boiling, add the barley with a pinch of salt. Cover, turn the heat down a little and simmer for 20-25 mins.
While the barley cooks, line a baking tray with foil. Rub 1 tsp oil onto the skin of each pollock fillet and lay them on the tray, skin-side up. Season with a little salt and pepper. Bake in the oven for 12-15 mins, till the fish is white and flakes easily when pressed with a fork.
While the fish cooks, warm a deep frying pan or wok over a high heat for 1 min. Add the shallots and mushrooms with ½ tbsp oil and a pinch of salt and pepper. Fry for 5-8 mins, stirring occasionally, till the shallots and mushrooms are golden.
Stir in the garlic and parsley stalks. Cook and stir for 1 min. Crumble in the stock cube and pour in 150ml boiling water. Pop on a lid and simmer for 5 mins till the mushrooms are tender. Stir in the spinach to wilt it. Take the pan off the heat.
The pearl barley should be tender by now. Drain it well. Spoon the pearl barley onto 2 warm plates. Spoon over the veg and their sauce. Lay a pollock fillet on top of each and garnish with some roughly chopped parsley leaves to serve.