In a wok or large skillet, heat 1 to 2 tablespoons of oil until it shimmers. Cook the pork, in batches if necessary, until it is lightly browned and cooked through. Remove from the oil and transfer to a plate.
Stir the onion in the wok or pan for 3 to 4 minutes, until soft and lightly browned. Place in a bowl and set aside.
Stir the mushrooms for 3 to 4 minutes, until they have exuded their liquid. Reduce the heat to medium. Combine with the onions and set aside.
In a wok or pan, cook the celery for 3 to 4 minutes, stirring occasionally, until crisp-tender. Combine the onions and mushrooms in a bowl.
Put the pork back in the wok or pan over medium heat. Mix in 1 1/2 cups chicken broth and remove any browned bits with a spatula.
Add the soy sauce, brown sugar, and brown gravy, followed by salt and pepper to taste. Cook, stirring, until the sauce bubbles at the edges, on medium-high heat. Stir the cooked vegetable mixture into the saucepan, and cook for 3 to 4 minutes. Season to taste.
Stir together the cornstarch and 1/4 cup of chicken broth in a small bowl. Continuously stir the cornstarch mixture into the gravy until it thickens. Pour over chow mein and Enjoy!