Fill a medium pan with boiling water. Add a pinch of salt and bring back to the boil, then add the pearl barley. Half-cover the pan and simmer the barley for 20 mins till it's tender but not too soft.
While the barley cooks, make the twistziki. Spoon the yogurt into a mixing bowl. Give the radishes a rinse (if your radishes came bunched, trim off the leaves – which are edible, and are all sorts of tasty rinsed and tossed into a salad). Finely slice the radishes and add them to the yogurt. Peel and crush or grate the garlic and scoop into the bowl.
Pick the soft feathery leaves off the dill sprigs and finely chop the leaves. Add most of them to the bowl (keep a pinch back for garnishing). Finely grate in the lemon zest and squeeze in the juice from half the lemon. Add a good pinch of salt and pepper and stir to mix. This is your twistziki.
Heat your grill to high and line the grill pan with foil (if your grill didn't come with a pan, use a baking tray). Rub 1 tsp olive oil into the skin of each salmon fillet. Season. Place skin-side up on the foil-lined grill pan or baking tray and grill for 8-10 mins or till the skin is crisp and the fish is cooked through and dark pink.
Toss the salad leaves with a squeeze of lemon juice and a pinch of salt and pepper.
When the barley is cooked, drain it and divide between 2 warm plates. Add the salmon and salad to the plates and serve with the radish twistziki, garnished with the last of the dill.