Cook the peas in simmering salted water for 3 minutes.Heat oil in a non-stick frying pan and sear the salmon on both sides until done.
For the dressing, finely chop the mint leaves and mix with the sugar, vinegar, olive oil and salt and pepper. Strain the peas, reserving a few spoonfuls of water. Mash the peas in the hot pan, then beat in the butter and salt and pepper with a wooden spoon. Gradually add a spoonful or two of the pea water, beating to a light puree. Beat in the mint vinaigrette.
Use a slotted spoon to server the pea puree, top with the salmon, and throw extra mint leaves on top.