Cut the beef into cubes. Marinate with 1 tablespoon oyster sauce, sweetened condensed milk, cornstarch, and black pepper. Mix well and let sit for 15 minutes. Smash the garlic and sauté until fragrant, then add the beef and stir-fry until just browned. Remove the beef and set aside.

Wash the bell pepper, apple, and onion. Cut them into cubes. Add them to the same pan and stir-fry with a pinch of salt. When the vegetables are just cooked, return the beef to the pan and stir everything together.

Season to taste and stir for another 2–3 minutes. Add chopped green onion (if you have celery or green onion tops, it will taste even better, but I used green onion because that's what I had). Turn off the heat.