Add all the batter ingredients to a clean bowl and stir until smooth and no lumps remain. Strain the mixture through a fine sieve to remove any remaining lumps.

For the sauce: Mix the sugar, soy sauce, potato starch, salt, and water together until well combined. Heat a nonstick skillet over medium heat, add a little oil, then add the ketchup and sauté until fragrant. Pour in the mixed sauce and bring to a boil, then transfer to a clean container.

Heat a nonstick skillet over medium heat and add oil. Add the oysters (make sure to drain them well to avoid splattering) and pan-fry briefly. Stir the batter before using, then ladle some over the oysters, swirling the pan to spread it evenly. When the edges start to lift, add the baby bok choy and eggs, then pour another ladle of batter over the top. After about 5–10 minutes, flip the omelette, pour another ladle of batter, and cook for another 5–10 minutes until done.