In a small bowl, whisk together soy sauce, vinegar, honey (or sugar), and olive oil. Season with freshly ground black pepper. Taste and adjust sweetness if needed.
Cut the tomatoes into bite-sized pieces. Slice the red onion thinly (or chop smaller if you prefer). Trim or roughly chop the watercress into shorter pieces. Lightly season with salt and pepper.
Pour the dressing over the vegetables and toss gently until everything is evenly coated.
Serve right away as a refreshing side dish.
Notes
Tomatoes: Use any kind you like—grape, pear, Roma, cherry, heirloom, or whatever’s in season. Just make sure they’re ripe and sweet so the salad tastes flavorful.
Watercress: Adds a fresh, peppery bite. If you don’t have it, arugula, baby spinach, parsley, or cilantro make great substitutes.
Extra Virgin Olive Oil: Healthier and adds a rich, fruity taste. You can also use regular olive oil, canola, or any mild-tasting oil.