If you are using powdered wasabi, place it in a mixing bowl and add 1 tablespoon of warm water. Stir to form a paste and let stand for about 5 minutes. If you are using wasabi paste, place it in a mixing bowl.

Grate the ginger on a fine grater into the bowl; you should have about 1 tablespoon.

Cut the lemon in half and cut a thin slice off one half. Cut the slice in quarters, remove any seeds, and set the lemon quarters aside for garnishing the sauce. Squeeze the juice from the remaining lemon into the bowl, squeezing it through your fingers to catch any seeds.

Add the soy sauce, scallion, and chile and stir to mix well. Divide the sauce among 4 small bowls. Float a quarter lemon slice in each bowl.