Heat your oven to 200°C/Fan 180°C/Gas 6. Crumble the stock cube into a heatproof jug. Add 750ml boiling water and stir to dissolve the stock cube.
Trim the top and bottom off the leek. Finely slice it. Warm a pan over a medium heat. Add ½ tbsp oil and the leek. Season. Cook for 5 mins, stirring often, till the leek has softened but doesn’t have too much colour. Turn the heat right down. Finely grate the zest from the lemon.
Tip the pearl barley into the pan. Add the lemon zest. Stir to mix it with the leek. Pour in the stock. Cover and simmer for about 30 mins till most of the stock has been absorbed. Stir now and then to stop the barley sticking.
While the pearl barley cooks, trim the rough ends from the fennel, keeping any fronds for later. Halve it and slice into around 8 wedges. Pop on a baking tray. Drizzle with 2 tsp oil and some seasoning. Rub the oil and seasoning into the fennel.
Roast the fennel for 20 mins till it’s starting to brown and soften. Add the salmon fillets to the tray, skin-side down. Roast for another 10 mins till the salmon is cooked and flakes easily when you press it with a fork.
Finely chop the leaves from the dill. Stir the dill into the pearl barley risotto. Squeeze in a little lemon juice. Taste the risotto and add more salt, pepper or lemon juice if you think it needs it. The barley should be tender and loose, with some liquid still in the pan.
Spoon the barley risotto into two warm shallow bowls. Top each one with a handful of rocket, the fennel wedges and salmon. Garnish with any fennel fronds you saved earlier. Serve straight away.