1. 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.
2. 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.
3. Toss the crabmeat with the remaining vinaigrette. Spoon the dressed crabmeat over the salads.
Wine Recommendation: Avocados are so rich it can be hard to find a wine to match. Opt for contrast instead by using a crisp Italian white wine such as Soave or Orvieto. Its fresh but neutral flavors will stand back and let the salad shine.
Notes: Wait until you're ready to toss the salad before dicing the avocados. The acidity in the vinaigrette will keep them from turning brown.