This is the way my mother taught me to cook ong choy. One important thing to keep in mind is that the stems contain quite a bit of moisture so make sure you cook on high heat and stir frequently so the moisture doesn't build up in the bottom of the wok.
Trim and discard the bottom 1 inch of the ong choy (it tends to be more fibrous) and chop the rest into 3-inch pieces. I usually leave the leafy parts a bit longer.
Place the ong choy in a large bowl and rinse under cold water a few times. I also like to let it soak for 15 minutes to remove any dirt inside the stems. Transfer to a colander or salad spinner to drain.
In a small bowl, combine the fermented tofu, sugar, and salt. Mash the tofu until the mixture is smooth.
Heat two tablespoons of oil in a wok over high heat. Add the garlic and cook for 10 seconds. Immediately, add the water spinach and cook for 30 seconds until it just starts to wilt. Make sure to stir frequently to allow steam to escape.
Add the sauce mixture and cook for another 1-2 minutes until cooked through. Enjoy!