To prepare the ingredients, thinly slice the char siu and Napa cabbage and cut the chives into 1-inch pieces.
To make the egg mixture, in a large bowl, whisk the eggs with the salt until smooth. In a small bowl, mix the cornstarch with the water to form a slurry, then stir it into the eggs. Add the char siu, Napa cabbage, bean sprouts, and chives and mix until evenly combined.
To fry the egg foo young, in a wok or large nonstick pan over high heat, add 1 tablespoon of cooking oil. Once hot, reduce the heat to medium-low and ladle in a portion of the egg mixture to form a small pancake. Cook for about 2 minutes per side until golden brown and cooked through. Remove and repeat with the remaining mixture, adding oil as needed.
To make the gravy, in a saucepan over medium heat, combine the chicken stock, soy sauce, oyster sauce, and sesame oil and bring to a simmer. In a small bowl, mix the cornstarch and water to form a slurry, then whisk it into the simmering sauce. Cook until thickened and glossy, then remove from heat.
To serve, place the egg foo young on a serving plate and generously drizzle the warm gravy over the top. Serve immediately.