Beef Rib Eye Roast With Red Wine Mushroom Sauce

• 4 mushroom (pieces)
• 1,400 gr beef rib eye (boneless)
• 120 ml dry wine (red)
• 60 gr onion (chopped)
• 8 gr cornstarch
• 2 gr salt
• 2 gr parsley (chopped)
• 1 gr pepper