Slice the cucumber into thin discs. If it is thicker than a nickel, you have sliced it too thick. Use a mandolin if you have one. Put them in a bowl and set aside. Slice the onion in long and semi thick pieces and mix into the bowl with the cucumber. Dice the tomato and add as well.
Make the dressing in a separate bowl by combining the soy sauce, rice vinegar, garlic powder sesame oil, white pepper and ginger.
Toss the salad with the dressing and let marinate for at least 30 minutes before serving.