In a small saucepan, combine the ketchup, soy sauce, oyster sauce, sugar, and 1/2 cup water.
Bring the mixture to a simmer over medium heat.
Stir in the cornstarch mixture and cook until the sauce thickens. Remove from heat and set aside.
In a small bowl, mix the sweet potato starch or tapioca starch with 1/2 cup water to create a smooth batter.
Heat 1 tablespoon of vegetable oil in a large non-stick skillet over medium-high heat.
Add the cleaned oysters and cook for about 1 minute until they start to release their juices.
Pour the starch batter over the oysters, spreading it out evenly in the skillet.
Add the chopped spinach or napa cabbage and green onions on top of the batter.
Season with salt and black pepper to taste.
Drizzle another tablespoon of vegetable oil around the edges of the omelette.
Beat the eggs in a bowl and pour them evenly over the omelette.
Cook for another 2-3 minutes until the eggs are set and the bottom of the omelette is crispy.
Carefully flip the omelette using a spatula and cook for another 1-2 minutes on the other side.
Transfer the oyster omelette to a serving plate.
Drizzle the prepared sauce over the top.
Enjoy your meal!