Make the sauce by combining the chicken stock, oyster sauce, sugar, and sauce in a small pan over medium heat. Cook until warm and bubbly.
Prepare cornstarch slurry by combining the cornstarch and water together in a small bowl. Stir to combine.
Add in the cornstarch slurry and stir until the sauce thickens. Turn off the heat.
Egg Foo Young
To make the egg foo young, in a large bowl add the eggs. Beat eggs together until the whites break down completely.
Stir in the cooked chicken or cooked shrimp, celery, mushrooms, sliced green onion whites, soy sauce, sesame oil into the egg mixture, and stir well.
In a heated wok add about a tablespoon of vegetable oil into the base. When the oil is hot stir it around coating the sides. Pour about a half cup of the egg foo young batter into the pan.
The eggs should bubble up when the batter is poured into the pan. Stir the uncooked eggs gently to the sides of the pan so they cook. The egg foo young will cook for about 2 minutes before you will need to flip the eggs to the other side.
Gently flip the egg foo young to the other side, and cook for another 2 to 3 minutes, or until this is a golden brown color.
Serve with brown gravy and top with green onion slices.