For Sauce

• -Melt the butter in a saucepan over medium heat, add the onions and stir for about 2 minutes.
• -Add the flour and stir for 1 minute, then add the stock, vinegar, and mushrooms. Season with salt and pepper. Mix well.
• -Leave the sauce over low heat for 5 minutes, or until thickened, then set aside.

  For Steak

• -Heat the oven to 180 degrees Celsius.
• -Season the meat with salt and pepper on both sides, and place onto an oven pan drizzled with some oil.
• -Place the meat into the oven for 20 minutes, or until cooked.
• -Cut the steak into strips, place onto the serving dish and garnish with the sauce, and fresh herbs.