In a blender, combine the onion with the vinegar, shallot, soy sauce and ginger and puree. Add the oil and blend until smooth. Transfer to a small bowl, add the cilantro and season with salt and pepper.
Cook the haricots verts in boiling salted water until barely tender, 5 to 7 minutes. Drain and rinse under cold water.
In a large bowl, toss the mesclun with the avocado, mango, bell pepper, cucumber, haricots verts and the dressing. Mound the salad on plates and serve.