After cleaning yu choy, shake out as much excess water as possible. Take handfuls and shake them out individually placing each handful in the wok cover. Then repeat, putting them back in the colander.
Heat oil in wok on medium/high heat until starting to smoke. (Note 1)
(Skip to the next step if not using garlic) Add minced garlic and toss until fragrant (about 20 seconds) making sure not to burn.
Place yu choy in 3-4 batches carefully in the wok. Flip and coat the leaves between batches. Leaves will immediately start to wilt allowing more yu choy to be added. (Note 2 & 3)
Once all yu choy is added, add a pinch of salt, soy sauce and sugar.
Sautee for another 30 seconds mixing thoroughly. Add broth/water, cover and lower heat to medium.
Steam for 3-5 minutes. Stir halfway through (for even cooking). (Note 4)
Sample a stem to check for doneness. Cook a bit longer if needed. (Note 5)
Add oyster sauce and sesame oil (if using). Toss to coat.