Preheat oven to 350 degrees F.
Place onions in a roasting pan, if desired. Place tenderloin in pan. Pour soy sauce and melted butter over the tenderloin.
Bake in preheated oven for 10 minutes, then turn the roast over and continue cooking for 60 minutes for medium rare, basting every 20 minutes.
If desired, put mushrooms around pan 20 minutes before baking time is done.
Let meat rest for 10-15 minutes before slicing.