Cut thick gai lan stems on a sharp bias into thin slices. For the thinner stems, chop them into 2-inch pieces. Cut the leaves into bite-sized chunks, keeping stems and leaves separated.

In a mortar and pestle, pound the garlic cloves until they're shredded. If you'd like extra heat, add the Thai chilies at this stage.

In a small bowl, mix together the oyster sauce, soy sauce, sugar, and water. Stir until the sugar dissolves to create a smooth marinade.

Heat up some oil in your trusted wok over medium-low heat. Add the pounded garlic and sauté until golden and fragrant, about 2 minutes—watch closely to avoid burning.

Add the gai lan stems to the wok and stir-fry for 30-45 seconds on medium heat. You want them to start softening but still crisp.

Increase the heat to high, then toss in the gai lan leaves, prepared sauce, and ground pepper. Stir everything together for another 30-45 seconds until the leaves are wilted but retain their vibrant green color.

Remove the wok from heat at this point to avoid overcooking the greens. They should be tender but still have a bit of bite left.

Serve immediately over a bed of fluffy jasmine rice, garnished with additional chilies or sesame seeds if desired.