Cut the chuck flap tail into bite-sized pieces.
Prepare the eggplant and onion, then slice them into even pieces.
Heat cooking oil in a pan and stir-fry the chuck flap tail over high heat.
Once the beef is mostly cooked, add the eggplant and onion and stir-fry.
Season with oyster sauce and soy sauce to taste.
Stir-fry until all ingredients are well combined and serve.