Season the Chicken: In a bowl, mix chicken thighs with soy sauce, vegetarian oyster sauce, and black pepper. Add a light coat of cornstarch and mix well to evenly coat.
Pan-Fry: Heat oil in a pan over medium-high heat. Add the chicken and cook for 2–3 minutes per side, until golden and about 80% cooked through. Remove and set aside.
Sauté the Onion: In the same pan, add sliced onion and sauté for 3–4 minutes until fragrant and slightly softened, letting it soak up all the savory chicken juices.
Add Sauce & Simmer: Pour in the stock, soy sauce, and mirin. Stir well, then add the chicken back into the pan. Let everything simmer for about 4 minutes so the chicken stays juicy and flavorful.
Finish with Eggs: Turn the heat down to low, Slowly pour in beaten eggs around the chicken. Cover with a lid and cook for 1–2 minutes, or until the eggs are just set and silky.
Serve: Spoon everything over a warm bowl of rice, making sure to get that glossy sauce on top.
