Drain capers and soak in a small bowl with warm water. Let sit while you mix and chop the rest of the ingredients.
Trim the stems off of the mushrooms. Clean and then slice very thin.
Place the mushroom slices in a mixing bowl with the finely sliced red onion. Mix carefully with your hands making sure not to break the mushrooms.
In a separate small bowl make the vinaigrette: combine the lemon juice, soy sauce, salt, sugar, mustard and pepper to taste. Whisk in the extra virgin olive oil.
Drain the soaking capers and mince finely. Add the capers to the vinaigrette. Taste and season as necessary.
Save 4-8 chives from the bunch to use as garnish. Then finely chop the remaining chives. Add to the vinaigrette.
Instead of tossing the mushroom/onion mixture with the vinaigrette, spoon on each of the four plates half of the mixture. Spoon half of the vinaigrette, divided evenly among four dishes, over the the spoonful placed on the plate. Then add the remaining half of the salad to each of the plates. Finally, top with the remaining mushroom/onion mixture, followed by the vinaigrette equally. Season with fleur de sel and lay 1-3 chives over each serving.