Make 2-3 cuts in steak and insert chopped garlic. Spread mustard on one side of steak.
Place steak in frying pan, mustard side down. Sprinkle some salt and pepper. Cook a few minutes on both sides. Take out steak and place on plate.
In the frying pan (with the steak juices), toss in onions and mushroom. Add some soya sauce and oyster sauce. Stir in diluted cornstarch to thicken sauce.
Pour sauce over steak. Add some cilantro garnish if desired.