Add the grated turnip and 1 cup water to a wok or large pan, and bring to a simmer. Simmer for about 10 minutes, stirring occasionally so the turnip does not brown. The turnip will produce liquid, some of which will evaporate. You will have liquid left in the wok with the radish that you will also use. Scoop the cooked turnip into a large mixing bowl to cool. Pour any remaining liquid in the wok into a measuring cup, and add water until you have a total of 1 cup of liquid. Add it to the mixing bowl with the cooked turnip.
Clean your wok or pan and place it over medium heat. Add a couple tablespoons oil. Add the shrimp, mushrooms, and sausage, and cook for about 5 minutes. Stir in the chopped scallion and remove from the heat to cool.
Add rice flour, cornstarch, salt, sugar, and white pepper to the mixing bowl with the radish and cooking liquid. Mix well until the dry ingredients are well-incorporated. Add in the cooked shrimp, mushrooms and sausage, and be sure to scrape the oil from the pan into the batter. Mix well and let sit for 15 minutes.
Give the batter a final stir, and pour it into a well-oiled loaf pan. Place the pan into a steamer with plenty of water and steam over medium-high heat for 50 minutes.
Remove the pan from the steamer and let your turnip cake set for about 30 minutes. Once cooled, loosen the sides with a spatula and turn it out onto a cutting board. It should come out quite easily.
Use a sharp knife dipped in water to slice ½-inch thick pieces. I know people who’d enjoy it just like that, but most people pan-fry the turnip cake first. Add a couple tablespoons oil to a non-stick or seasoned cast iron pan over medium heat. Fry the cakes on both sides until golden and crispy. Serve with oyster sauce!
Note: You can also make this turnip cake in advance, refrigerate it in the loaf pan, and slice/fry later. If you’ve refrigerated turnip cakes that you’ve already fried, the best way to reheat them is to re-fry them in a pan.