In a small bowl or measuring cup, whisk together sauce ingredients: soy sauce, rice wine vinegar, brown sugar, water, grated ginger, garlic, cornstarch, and sesame oil. Set aside.
Heat a large frying pan or a wok over medium-high heat; add 1 tablespoon oil. Add broccoli and onion, sprinkle with ¼ teaspoon salt, and cook for 6-7 minutes, stirring frequently, until broccoli is tender-crisp.