Cut bell red pepper in half lengthwise. Remove cores and seeds. Cut into 1 inch strips.
Cut yellow onion into even slices.
Cut green onions in half. Cut the top into 1 inch. Mince the remaining well.
Mince garlic cloves. Set aside.
Slice beef across the grain thinly. Place the beef into a bowl. Add 1/2 tsp. sugar, 1/2 tsp. mushroom seasoning, 1/4 tsp. ground black pepper, 1 tsp. soy sauce, 1 tsp. oyster sauce, and minced onion. Mix until well combined. Marinate for 10 minutes at least.
Making the Sauce
In a small bowl, combine 1 tsp. sugar, 1/4 tsp. salt, 2 tsp. soy sauce, 2 tsp. oyster sauce, and 2 tsp. water. Stir until dissolved.
In another small bowl, add 1 tsp. corn starch and 3 tsp. water. Stir until smooth. ( This will thicken the sauce by the time we add it)
Heat 1 tsp. vegetable oil in a skillet over medium heat. Add half amount of minced garlic. Stir until it turns light brown. Add marinated beef and stir fry until medium rare. Transfer the beef into a clean bowl.
Use that pan. Add 1 tsp. vegetable oil and remaining garlic. Stir until fragrant. Add green beans and stir for a few times. Then add 3 tsp. water. Cover the top and cook for 3-4 minutes.
Take off the lid. Add pineapple, bell red pepper, and yellow onion. Pour in the sauce. Stir them up.
Once again, cover the top and cook for another 3 minutes. Then Remove the top. Add green onions. Stir well. Then add beef. Stir and cook for 1 minute.
Pour in the corn starch mixture. Stir quickly for 2-3 seconds. Remove from heat immediately.
Transfer the vegetable into a dish. Sprinkle with ground black pepper. Serve Hot!