Using the pan that the tenderloin was cooked in, place it over a medium high heat and begin to gently sauté the diced onions in the oil that is already in the roasting pan, cook for one to two minutes.
Add the butter salt and pepper and continue cooking, add a splash of white wine to moisten, sprinkle with chopped parsley and give one final toss
Place the mushrooms on the bottom of a warm serving platter, take an hour-rested beef tenderloin and present whole on top of the mushroom, or, sliced and laid over the top of the mushroom.
Serve with the Brussel sprouts and creamy scallop potatoes.
