Wash the lettuce heads thoroughly. Separate leaves and gently pat dry with towels or use a salad spinner. Trim any tough stem at the base so leaves lie flat; keep whole large leaves for serving.

Make sauce

In a small bowl combine oyster sauce, light soy sauce, sugar, sesame oil, and the 3 tablespoons of stock or water. Stir to dissolve sugar. In a separate tiny bowl mix cornstarch with 1 tablespoon cold water to make a smooth slurry; set aside.

Cook garlic

Heat a large skillet or wok over medium-high heat. Add vegetable oil and swirl to coat. Add the thinly sliced garlic and stir-fry 20–30 seconds until fragrant and just starting to color — do not burn.

Add lettuce

Immediately add the drained lettuce leaves to the pan. Using tongs or a spatula, toss gently to coat the leaves with oil and garlic. Stir-fry for 30–60 seconds until leaves begin to wilt but remain bright green and slightly crisp.

Add sauce and thicken

Pour the prepared oyster-soy mixture into the pan, add the cornstarch slurry, and toss gently to combine. Cook 30–45 seconds until the sauce thickens slightly and coats the leaves. If sauce looks too thick, add another tablespoon of stock or water.

Finish and season

Season with salt and freshly ground black pepper to taste. Remove from heat while leaves still have some crispness. Transfer to a serving plate, spoon any remaining pan sauce over the lettuce to glaze.

Garnish (optional)

Scatter toasted sesame seeds, sliced scallions, and thin chili slices over the glazed lettuce. Serve immediately as a warm vegetable side.