How to Make It
In a blender, combine the ginger, lemon zest, lemon juice, vinegar, scallions, soy sauce, and 1/4 teaspoon of the salt. Pulse to chop. Add the oil and puree until smooth.
In a large glass or stainless-steel bowl, combine the watercress, romaine, avocados, and the remaining 1/4 teaspoon salt. Toss the salad with all but 3 tablespoons of the vinaigrette and mound the salad on plates.
Toss the crabmeat with the remaining vinaigrette. Spoon the dressed crabmeat over the salads.
Wait until you're ready to toss the salad before dicing the avocados. The acidity in the vinaigrette will keep them from turning brown.