Rinse 1 lb choy sum thoroughly under cold running water to remove grit, especially at the base and between stalks. Trim off any tough root ends and separate stalks if bundled. If stalks are large, cut them in half lengthwise. Peel and julienne enough carrot to make 1/2 cup. Peel and thinly slice or smash 3 garlic cloves.

Make sauce

In a small bowl combine 3 tablespoons oyster sauce, 1 tablespoon light soy sauce, 1/4 cup chicken or vegetable stock (or water), 1 teaspoon sesame oil, 1/2 teaspoon sugar, 1/4 teaspoon salt and a pinch (about 1/16 tsp) white pepper. In a separate small cup, mix 1/2 teaspoon cornstarch with 1 tablespoon water to make a slurry; set both aside.

Cook

Heat wok

Place a wok or large skillet over high heat until very hot (about 2 minutes). Add 1 tablespoon neutral oil and swirl to coat.

Aromatics

Add the sliced or smashed garlic and stir-fry for 10–15 seconds until fragrant and just beginning to color. Be careful not to burn.

Stir-fry vegetables

Add the choy sum stalks first and stir-fry for 30–45 seconds, then add the leafy tops and the julienned carrot. Continue stir-frying, tossing constantly, for another 30–45 seconds until stalks are bright and crisp-tender. If the pan becomes dry, splash 1–2 tablespoons of the reserved stock/water.

Add sauce and thicken

Pour the prepared oyster sauce mixture over the vegetables and toss to coat. Immediately stir in the cornstarch slurry and cook for 15–30 seconds, stirring, until the sauce slightly thickens and becomes glossy and clings to the vegetables.

Finish and adjust seasoning

Taste and adjust seasoning if needed — add up to another 1/2 teaspoon salt or a splash of soy if you prefer saltier. Turn off heat and drizzle 1 teaspoon sesame oil over the vegetables and give a quick toss.

Serve

Transfer the choy sum with oyster sauce to a serving plate. Garnish with toasted sesame seeds and/or a sprinkle of fried garlic or chili flakes if using. Serve immediately alongside steamed rice.