Cut the tempeh into large cubes, put in a medium saucepan, and cover with water. Bring to a boil, then simmer for 10 minutes. Drain and set aside to cool while you prepare the rest of the salad.
In a medium bowl, place the shallot, green onions, and celery. In a smaller bowl, whisk together the mayonnaise, mustard, lemon juice, dill, salt, and pepper, and add to the bowl with the vegetables. Use your hands to crumble the cooled tempeh into the bowl, then mix everything together. Sprinkle the nori flakes over the salad, then mix again. Taste, adjust seasonings to your liking, and then serve or chill. The salad will keep in the refrigerator for up to five days.