In a mixing bowl, add eggs, sesame oil, ½ tbsp corn starch slurry, 1 tsp soy sauce, ½ tsp pepper. Beat well.
Make the sauce: In a bowl, mix 1 tbsp oyster sauce, 1 tbsp soy sauce, brown sugar, sweet say sauce, ketchup, ½ tbsp corn starch, broth. Put the mixture into a small pot over low heat and cook until it thickens.
Stir fry onions and chicken in a flat cast iron or stainless-steel pan.
Add the carrot, celery, spring onions, black pepper, salt, 1 tbsp soy sauce, 1 tbsp oyster sauce.
Add egg mixture into the pan and turn the heat down.
After a few minutes when the underside is cooked, carefully fold the omelet into half in the pan.
Cut the folded omelet into three portions, flip as necessary to make sure both sides are cooked through. Disk them out onto serving dish.
Pour the sauce over the omelet.