Place beef strips in a bowl and add marinating ingredients. Marinate for at least 30 minutes.
600 grams flank steak, 1/4 tsp salt, 1/4 tsp baking soda, 1/4 tsp ground black pepper, 2 tbsp potato starch, 1 egg white
While beef is marinating, make the sauce by mixing all ingredients in a small bowl. Set aside.
1/2 cup water, 1 tbsp light soy sauce, 1/4 cup hoisin sauce, 2 tbsp oyster sauce, 2 tbsp sugar, 2 tbsp hot sauce, 2 tbsp ketchup
Heat vegetable oil in a wok or pan over medium-high heat. Once oil is hot enough, lightly dredge marinated beef in cornstarch and fry beef slice in batches for 3-4 minutes. Once all meat are done frying, set aside.
2/3 cup vegetable oil
In another wok, place and heat 2 - 3 tablespoon of oil from frying the beef over medium heat. Sauté garlic, onion until fragrant and translucent.
1 medium onion, 2 cloves garlic
Add bell peppers. Once bell peppers have wilted, tip in the fried beef slices and stir fry for a few seconds.
1 large red bell pepper
Pour the Beijing beef sauce and gently toss the ingredients until coated with the sauce. Add vinegar and let simmer for a minute without stirring.
1 tbsp vinegar
Add thickening agent and stir until sauce is thick and sticks to the fried beef slices. Remove from heat and serve while warm.
1 tbsp cornstarc dissolved in 1 tbsp water