Marinate the thinly sliced beef with light soy sauce and cornstarch. Let it sit for 15 minutes while you prepare the other ingredients.

In a bowl, mix together the ingredients for the black pepper sauce: dark sweet soy sauce, tomato paste, oyster sauce, black pepper, and water. Make sure to combine them well.

Heat a tablespoon of oil in a pan over medium-high heat. Once hot, add the marinated beef slices and cook until they are seared, about 3-4 minutes. Make sure not to overcrowd the pan for even cooking.

Add the minced ginger, sliced onions, and minced garlic to the pan. Stir-fry until fragrant, about 1-2 minutes. Be careful not to burn the garlic.

Toss in the sliced capsicum (bell pepper) and the black pepper sauce from the bowl. Stir everything together and cook until the sauce starts bubbling, about 2-3 minutes.

Once the sauce bubbles, add the cornstarch slurry. Stir well and cook until the sauce thickens, about 2 minutes. This will give your dish a nice glossy finish.