Put the strips of salmon in a non-aluminium saucepan or frying pan with a well-fitting lid. Arrange the scallops on top of the salmon. Pour in the wine and stock and add the slice of ginger. Bring to the boil over a medium heat, then reduce the heat until the liquid is simmering gently. Cover and poach for 5–8 minutes or until the salmon and scallops are cooked and feel just firm to the touch.
While the seafood is cooking, drop the sugar snap peas into a saucepan of boiling water and cook for 3–4 minutes or until just tender but still crunchy. Drain, then refresh under cold running water. Set aside until needed.
To make radish flowers, cut some slits around each radish, cutting down from the top almost to the base. Put the flowers in a bowl of iced water and leave until the ‘petals’ open slightly. Alternatively, simply slice the radishes.
Put the mixed leaves in a salad bowl. Add the sugar snap peas and the drained radishes and mix well.
With a slotted spoon, lift the seafood out of the pan onto a plate, reserving the poaching liquid. Cut each strip of salmon in half, or flake it into large chunks. Arrange the salmon and scallops on top of the salad.
To make the dressing, remove the tough stalks from the watercress and reserve. Drop the leaves into a saucepan of boiling water and bring back to the boil. Immediately drain and refresh under cold running water. Squeeze out any excess water, then chop finely.
Put the reserved watercress stalks in a saucepan with the shallot, lemon zest and 1/2 cup (125ml) of the fish poaching liquid. Half cover the pan and simmer for 5 minutes. Strain, discarding the solids. Stir in the watercress leaves, chives, lemon juice and yogurt, then season to taste with salt and pepper. Spoon the warm dressing over the salad, then serve.