Combine oyster sauce, 2 tsp cornstarch, 1/4 tsp black pepper, baking powder in a bowl. Mix well and add steak. Let the mixture marinate for at least 10 minutes.
Combine broth, rice wine, dark soy sauce, sugar, and remaining 1/4 tsp black pepper in a small bowl to make sauce.
Place a wok over high heat until hot. Add 1 1/2 tbs oil, swirling to coat sides. Add garlic and ginger to cook until fragrant. Add beef and stir-fry until no longer pink, about 1 1/2 to 2 minutes. Remove meat from wok.
Add remaining 1/2 tbs oil to wok, swirling to coat sides. Add bell peppers and stir-fry for 1 minute then add the 2 tbs water.
Stir-fry for another 2-3 minutes until tender-crisp.
Return meat to wok and add sauce. Bring to a boil then add cornstarch solution, stirring, until boils again and thickens.