Steam the beans over boiling water for 5 minutes, then rinse with cold water.
Mix the dressing and pour half of it over the beans. Mix well and set aside.
Shred the salmon into thin, long slices.
Cut the avocado in half, remove the stone, then cut the flesh into slices or chunks. Immediately squeeze some lemon juice over them.
Scatter the watercress onto a large plate. Add the beans, then arrange the avocado and salmon on top. Season with freshly ground pepper.
Drizzle with the remaining dressing, then scatter the sesame seeds on top.