My first attempts to cook with water spinach (also called ong choy, kangkung, rau muong) ended in failure: the veggie would rot before I got around to cooking it! Of course, old hands know that water spinach goes bad very quickly, so I would encourage you to buy your bunch the day-of and refrigerate it. Either way, water spinach is a wonderful vegetable - despite it's name, I don't think it tastes anything like spinach. The hollow stem gives it a light crunch, and flavor-wise it's very neutral.
There are numerous ways to prepare this veggie (a Chinese method is a classic saute with garlic, soy sauce, oil & sherry), but I love the pungent Malaysian preparation, which uses shrimp paste.
1/4 cup onions, finely sliced
1 clove of garlic, minced
handful of small dried shrimp, soaked & minced
1 red chili, sliced
1 tbsp shrimp paste (belacan)
bunch of water spinach (kangkung)
dash of soy sauce
Prep the water spinach by washing thoroughly, then cutting off stems & chopping roughly. Set aside.
Saute garlic & onions with cooking oil until translucent in a large pan or wok on medium-low heat.
Drain and mince the dried shrimp. Add the shrimp into the stir fry. Chop the red chili very thinly, removing seeds, and add the chili and shrimp paste (belacan) into the wok. Stir around for a few minutes.
Turn the heat up, then add the chopped water spinach. Stir rapidly, ensuring the water spinach gets cooked evenly. Add soy sauce, taste, and add salt if you like. Remove from heat after a few minutes, when the kangkung looks just wilted -- don't overcook!