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Heat oven to 220C/200C fan/gas 7. Put the radishes and potatoes in a shallow roasting tin. Drizzle with 2 tbsp of the olive oil and season. Roast in the oven for 30 mins, tossing once, until tender and golden.
Meanwhile, make the dressing. Whisk together the remaining oil, the yogurt, lemon juice, garlic, chopped dill and seasoning. It may separate slightly and need whisking again before serving.
Remove the veg from the oven and allow to cool for 5 mins. Toss the lettuce in the dressing, top with the veg and then the fish. Sprinkle over the reserved dill fronds and serve.