Trim and slice the chicken. Pat the breasts dry with paper towels (removing surface moisture gives better sear). Slice the chicken across the grain into 1/4-inch (6 mm) thin strips so pieces cook quickly and remain tender. Cutting across the grain shortens muscle fibers, making the meat feel more tender when you bite it.

Marinate the chicken: in a medium bowl toss the sliced chicken with 1 tsp kosher salt, 1/4 tsp black pepper and 1 tbsp cornstarch. Let rest for 10 minutes while you prep aromatics. The light cornstarch coating performs two jobs: it protects the surface from overcooking (velveting effect) and creates a glossy, silky exterior that helps sauce cling.

Make the sauce: in a small bowl combine 3 tbsp low-sodium soy sauce, 1 tbsp rice vinegar, 1/3 cup (80 ml) chicken broth, 1 tsp sugar and the remaining 1 tbsp cornstarch. Stir to dissolve the sugar and cornstarch — this will be your sauce base and thickener. Pre-mixing prevents lumps and ensures the sauce will thicken quickly when added to the hot pan.

Prep aromatics and mise en place: thinly slice 3 garlic cloves and cut the medium onion into 1/2-inch (1.2 cm) wedges. For a fast stir-fry everything must be ready before the pan goes on high heat; cooking at high temperature is quick and unforgiving.

Heat the wok or large skillet: place a 12-inch wok or heavy skillet over high heat until very hot (a drop of water should sizzle and evaporate instantly). Add 2 tbsp neutral oil and swirl to coat. Getting the pan and oil extremely hot ensures immediate searing and short cooking time, producing Maillard flavor without drying the chicken.

Sear the chicken in batches: add half the chicken in a single layer and let it sit untouched for 45–60 seconds to develop color, then toss and push to the side to finish cooking (~1–2 minutes). Remove to a plate and repeat with the remaining chicken. Working in batches prevents overcrowding, which would steam the meat instead of searing it.

Sauté aromatics: if the pan is dry, add the optional 1 tsp neutral oil, then add the sliced garlic and sauté 15–20 seconds until fragrant (don’t brown). Add the onion wedges and stir-fry 1–2 minutes until they begin to soften and pick up a little color. Cooking aromatics briefly on high heat unlocks their flavors without turning them mushy.

Return the chicken and add the sauce: return all the chicken to the wok and pour the pre-mixed sauce over everything. Use tongs or a spatula to toss so the sauce coats the pieces. The hot pan will activate the cornstarch in the sauce, giving a glossy sheen and slightly thickening in about 1–2 minutes.

Finish and balance flavors: when the sauce thickens and clings to the chicken, cook another 30–60 seconds to ensure the chicken is cooked through (internal temp 165°F / 74°C). Turn off the heat, stir in 1 tsp sesame oil for aroma, and taste — add a pinch of salt or a splash more soy if it needs more umami, or a squeeze of lime if you like brightness. Finishing oils add immediate aroma that you lose if added too early.

Rest briefly and serve: transfer to a warm platter and let rest 2 minutes; the sauce will settle and thicken just slightly more. Serve over steamed rice or noodles. Resting lets carryover heat finish the cooking and allows the juices and sauce to redistribute for a juicier bite.