In a small bowl, whisk together soy sauce, rice vinegar, vegetable broth, and cornstarch. Set aside.
- Heat a wok or skillet over medium-high heat. Add a splash of water or sesame oil if using.
- Stir-fry onions, garlic, and ginger for 1–2 minutes until fragrant.
- Add bell peppers and cook for 4–5 minutes, until tender-crisp.
- Pour in sauce mixture and stir well, cooking until thickened and vegetables are coated.
- Season with red pepper flakes or chili if desired.
- Garnish with scallions and serve hot with steamed rice or noodles.