Prepare egg mixture: In a bowl, mix all-purpose flour with 2 tablespoons of water, stirring well to avoid clumps. Add eggs and chicken powder, then whisk until well combined.
Prepare aromatics: Mince the garlic cloves. Slice the onion and green onion thinly.
Make sauce mixture: In a separate bowl, combine 250 ml water, oyster sauce, soy sauce, sugar, and cornstarch. Stir well until the cornstarch is fully dissolved.
Cook eggs and aromatics: Heat oil in a pan until hot. Pour in the egg mixture and scramble gently to form larger soft chunks. When eggs are set but not crisp, add garlic and onion. Cook until fragrant and slightly browned.
Add sauce and finish cooking: Pour the prepared sauce mixture into the pan. Bring to a boil and cook until the sauce thickens. Add green onion, mushroom bouillon, and sesame oil (optional). Cook briefly to combine flavors.
Serve: Serve the Eggs in Oyster Sauce hot, ideally with white rice.