Remove and discard yellow, damaged, or tough leaves from 1 pound Asian greens 4 to 8 inches long, such as baby bok choy, choy sum, yao choy, or small Chinese mustard greens. If head or stalk is thicker than 2 inches at the base, cut lengthwise into halves or quarters to make about 1 inch thick. Immerse in water and swish to dislodge grit. Drain.
Set a 5- to 6-quart pan over high heat. When hot, add 1/2 cup water and the greens; cover and cook until barely tender to bite, 3 to 5 minutes. Drain greens and pour into a serving bowl.
If desired, drizzle with 2 tablespoons oyster sauce or soy sauce and 2 to 3 teaspoons toasted sesame oil; sprinkle with about 1 tablespoon toasted sesame seeds.