Cook udon according to package instructions. Rinse under cold water and drain thoroughly.
Dice tomato, julienne cucumber, and chop green onions. Cut nori into small squares with scissors.
Combine all "A" ingredients in a microwave-safe bowl. Microwave uncovered at 600W for 1 minute 30 seconds. Stir, then microwave for another 1 minute 30 seconds. Stir again until evenly cooked.