Combine the chicken broth, soy sauce, rice vinegar, brown sugar, and sesame oil in a small saucepan. Place the saucepan over low heat, stirring occasionally.
In a small bowl, mix the cornstarch and cold water to create a slurry. Gradually whisk the slurry into the saucepan, continuing to stir until the gravy thickens and bubbles. Remove from heat and set aside.
For the Egg Foo Young:
In a large mixing bowl, whisk the eggs until lightly beaten. Add the red bell pepper, celery, mushrooms, onion, bean sprouts, half of the green onions, salt, white pepper, and cooked chicken. Stir to combine thoroughly.
Heat ½ tablespoon of canola oil in a nonstick skillet over medium heat. Pour approximately ½ cup of the egg mixture into the skillet, spreading it evenly. Allow it to cook for 2–3 minutes or until the edges are set and the bottom is golden brown.
Carefully flip the omelet using a wide spatula and cook the other side for an additional 2–3 minutes until fully set. Remove from the skillet and keep warm. Repeat the process with the remaining batter, adding more oil as needed.
Assembly and Serving:
Arrange the cooked Egg Foo Young patties on a serving plate. Drizzle generously with the prepared gravy.
Garnish with the remaining green onions and serve immediately.
Notes
To make this recipe gluten-free, substitute tamari for soy sauce and ensure the chicken broth is certified gluten-free. Always verify labels for hidden sources of gluten in condiments and stock.