In a small bowl whisk together; hoisin sauce, soy sauce, brown sugar and water. Set aside until you need it.
Slice steak into about 1-1/2" strips, place in a large bowl and toss with salt, pepper and cornstarch.
In a pan/skillet heat about 4 Tbsp. vegetable oil. Add in steak searing it on all sides, then remove from the pan.
Add a little more oil and add pepper flakes, ginger and garlic. Allow to cook for about 30 second to 1 minute. Add the beef back into the skillet, then the onions, and sauce we made earlier. Stir coating everything completely and allow to cook for about 1 minute.
Serve over rice and enjoy.