Cut the shiitake into chunks. Thinly slice the spring onions.
Shred the salmon into thin, long slices.
Heat the olive oil in a wok. Add the mushrooms, stir for a couple of minutes, then add the vinegar and soy sauce.
Add the spring onions, cover and turn down the heat. Cook for about 5 minutes or until the mushrooms are tender. Add the sesame oil and seeds, then set aside.
Meanwhile, cut the avocado in half, remove the stone, then cut the flesh into chunks. Squeeze some lemon juice over them immediately to prevent them from browning.
Scatter the watercress onto a large plate.
Add the stir-fried mushrooms, then arrange the avocado and salmon on top. Season with freshly ground pepper.
Drizzle with the some lemon juice, then scatter the sesame seeds on top.
Serve at room temperature.