Prepare the ingredients before cooking.
Heat the oil in a large pan or wok over medium-high heat.
Add the sliced beef to the pan, seasoning with some salt and pepper.
Toss in the minced garlic and sliced onions; sauté until fragrant.
Add the bell pepper and cook for an additional minute.
Stir in the soy sauce and oyster sauce, mixing them well.
Pour the cornstarch slurry into the pan, stirring to thicken the sauce.
Taste and adjust seasoning, adding more salt or pepper as necessary.
Remove from heat and serve immediately.
