Prepare the sauce by mixing soy sauce, hoisin sauce, garlic and red chile paste, Chinese hot mustard, rice wine vinegar, chili oil, brown sugar, minced garlic, minced fresh ginger, and red pepper flakes in a bowl.

Julienne carrots and celery.

Place beef in a bowl, coat with cornstarch, and let sit for 10 minutes.

Heat oil in a large skillet or wok.

Stir-fry beef until crispy (3-5 minutes). Remove and drain.

Drain excess oil from the pan, if any.

Add sesame oil to the pan, heat, then add celery, crushed red pepper flakes, and carrots, stirring between additions.

Stir-fry for 30 seconds.

Add the fried meat and green onions.

Pour the sauce over the ingredients.

Bring the sauce to a boil and cook for 1 minute.

Serve immediately over rice.