Cut the mushrooms in half, if they are big. Chop the spring onions.
Heat the oil in a small frying pan and sauté the mushrooms and spring onions, under cover, for 5 minutes. Add the soy, lemon juice and the vinegar. Put the cover on and sauté for another 5 minutes, until the mushrooms have shrunk. Scatter the sesame seeds on top and set aside, keeping it warm.
Whisk two eggs in a small bowl. Add the soy sauce and some chopped chives, if liked.
Heat a spoonful of oil in a flat frying pan and add the egg, letting it spread out. When the surface is no longer runny, spoon half of the mushrooms on one half, then lay two slices of smoked salmon on top. Drizzle with some lemon juice. and dust with freshly ground pepper, then scatter half of the rocket leaves on top.
Turn over the other, empty half of the omelette and cook for a few more seconds, then slide it to a plate and cut it into half.
Make another omelette, repeating the above steps.