Whisk cornstarch and water together in a small bowl until fully dissolved.
Combine soy sauce, minced garlic, sesame oil, brown sugar, and rice vinegar in a bowl. Whisk until sugar dissolves.
Stir the cornstarch slurry into the sauce mixture.
Heat wok or skillet over high heat. Add cooked protein and vegetables, push to sides, pour sauce into center and stir 30–45 seconds until glossy and thickened. Toss everything together and serve immediately.
Notes
Always use fresh garlic — jarred garlic loses the sharp aroma this sauce depends on. Store in a sealed jar in the fridge for up to 7 days. Freeze in ice cube trays for single-serving portions — lasts up to 3 months.