Preheat the oven to 180°C and line a baking tray with foil.
Boil the potatoes in salted water for 15 minutes or until tender, then drain and quarter.
Arrange salmon skin-side down on the tray, drizzle with half the lemon juice, season, and seal in a foil parcel.
Bake the salmon for 15 minutes until cooked through, then cool and remove the skin.
Whisk olive oil, mustard, sugar, and vinegar in a large bowl.
Toss the hot potatoes in the dressing, season, and leave to cool.
Mix in mayonnaise, spring onions, radishes, celery, and the remaining lemon juice.
Fold in half the parsley and transfer the potato salad to a platter.
Top with flaked salmon, remaining parsley, and peeled tiger prawns to serve.