In a bowl mix together 2 tablespoons vegetable oil, soy sauce, 2 tablespoons balsamic vinegar, sugar, garlic, Dijon mustard, salt, and pepper. Pour into a zip lock bag. Add the steak pieces and mix to coat. Seal the bag and refrigerate and marinate for at least 1-4 hours.
Heat oil in a large frying pan over medium-high heat. Remove the steak from the marinade and add to the pan, in a single layer and sear until browned on both sides and the steak is cooked to your desired doneness. Transfer to a serving platter and cover loosely with foil.
Add the mushrooms to the pan and cook, stirring occasionally for about 5 minutes. Add the vinegar and cook for 1 minute. Remove from the heat, add butter, and stir until melted. Pour over the steak, top with parsley and serve.