Heat the peanut oil in a wok or large skillet over high heat. Toss in the smashed garlic and cook for about 40 seconds, until fragrant.
Add the water spinach. Watch as the large bunch wilts and shrinks. Pour in the water and cover with a lid for 1 minute for stalks to steam through.
Add the sesame oil, chili flakes if using, salt and pepper. Toss with a spatula or a flick of the wrist. Transfer to a plate serve hot with your other mains and sides.