Preheat the oven to 200°C/fan 180°C/gas mark 6. Put the salmon fillets on a baking tray and cook according to the instructions on the packet.
Meanwhile, peel the cucumber into long thin strips using a vegetable peeler, and discard the watery core. Slice the radishes into rounds.
Combine the yogurt, lemon juice and dill with a little seasoning. Toss together the salad leaves, cucumber, radishes and mint leaves with almost all the dressing. Transfer to a large serving platter. Flake the salmon on top and add the remaining dressing and spring onions.