Prepare the sauce by whisking together soy sauce, oyster sauce, chicken stock, cornstarch, and water in a medium bowl. Set aside.

Marinate the beef by tossing sirloin tips with a pinch of salt and a splash of the sauce mixture, allowing to sit for 5 minutes.

Sear the beef in a hot wok or skillet with vegetable oil, cooking for about 2 minutes until browned.

Cook the vegetables in the same pan, stir-frying bell peppers and onion for 2 minutes, then adding garlic and ginger for another 2-3 minutes.

Combine the prepared sauce with the cooked vegetables, stirring for 2 minutes until thickened, then mix in the beef.

Finish by topping with green onions and optional hot sauce, and serve over rice or noodles.