Preheat oven to 260°C.
Boil the water in a kettle, then pour into a large roasting pan. Add the white wine and the lemon slices. Place a rack on the bottom of the pan and the fillets on top, then cover with a tight-fitting lid or foil.
Steam for 15 minutes.
While steaming, prepare the salad. Cut the peels off 3 oranges, top and bottom first. Then cut off the rest of the peel in strips around each one. For fancier preparation cut the segments out, otherwise just cut in half, then into slices. Add to a bowl with the sliced spring onions, watercress and rocket, and season.
In a saucepan, steam the snap peas for 5 minutes, then drain.
In a small bowl, mix the dressing ingredients together and whisk. Pour half onto the salad.
Remove the fish from the oven and place in the middle of each plate. Place the snap peas around the fish as well as a few orange pieces. Place the salad over the top of the fish and drizzle more of the dressing around the plate.