This is my go to meal for "Date Night". It sounds complicated but it is so easy. When you buy the beef tenderloin, just ask your butcher to butterfly the beef tenderloin by cutting the beef lengthwise down the center to within 1/2-inch of the other side.
My butcher is great and never has a problem doing it.
In a medium skillet, melt the butter and cook the shallots or onion over low heat until tender, about 5 minutes. Add the mushrooms and cook over low heat about 5 minutes or until the liquid is reduced by half. Let cool.
Butterfly the beef tenderloin by cutting the beef lengthwise down the center to within 1/2-inch of other side. Set the beef aside.
To stuff the tenderloin, open the beef and sprinkle in the inside with salt and pepper and rub with soy sauce. Spoon the mushroom mixture down the center of the tenderloin. Bring the 2 sides of the tenderloin up and around the filling to meet. Using butcher string, tie the tenderloin together at 2-inch intervals. Place the tenderloin in a roasting pan.
Roast for 40 minutes for medium rare. Let stand for 10 minutes before slicing. Slice about 1 1/2 inches thick.