Add oil to a small saucepan over medium heat. Fry onion, garlic and mushrooms until the onion browns on the edge.
Pour in water and bring to a boil. Cover with a lid and leave to simmer over low heat for 3 minutes.
Uncover then add light soy sauce, dark soy sauce, oyster sauce and sugar. Once it comes back to a boil, remove from the heat.
Pass the sauce through a sieve to remove all the solid bits. Set aside.
Mix the batter
Put rice flour, wheat starch (or cornstarch/tapioca starch), salt and oil into a mixing bowl. Add ⅓ of the water and stir until smooth. Pour in the rest of the water and mix well.
Steam the rolls
Brush a thin layer of oil over a small, light baking tray (sheet pan) that fits in the steaming apparatus you plan to use (see note 3).
In a wok/pot which you use for steaming, bring water to a boil. Place the tray over the steamer rack (or inside the steamer basket). Pour in batter just enough to thinly cover the surface of the tray. (I use about 60ml/¼ cup of batter for a 18cm×18cm/7”×7” tray).
Add the filling of your choice (see note 4 if using shrimp). Cover with a lid then leave to steam over medium heat for 3 minutes. You should see bubbles appear at that moment.
Uncover and carefully take out the tray. Leave to cool for about 1 minute.
Use a scraper (or a spatula) to gently lift the sheet from one end and roll it towards the other end. Repeat to steam the rest of the batter (Tip: to speed up the process, use 2 trays to rotate).
Serve and store
Transfer the steamed rolls to a serving plate. Use the scraper to cut them into bite-sized pieces. Pour the sauce over (or have it on the side for dipping).
You may store the leftovers (in an airtight bag/container) in the fridge for up to 3 days. Steam for 3 minutes to reheat. Alternatively, freeze for up to 1 month. Steam for 6 minutes without defrosting.
Video
Notes
1. This recipe makes about 150ml (⅔ cup) of sauce. It may be more than you need for 6 rolls. You can store it in the fridge for up to a month. You may use it as a dipping sauce for dumplings, spring rolls, turnip cake, etc, or as seasoning for fried rice, fried noodles, etc.2. Alternatively, use Cantonese sweet soy sauce and/or chilli oil as dipping sauce.3. Light-weighted, non-stick, square (or rectangle) baking trays/sheet pans are preferable. This recipe also works for special Cheung Fun makers.4. If using raw shrimp as the filling, you need to steam the batter for 1 minute then place the shrimp over and cook for another 2 minutes.