With the rack in the highest position, preheat the oven’s broiler.
On half of a baking sheet, toss onion and mushrooms with oil. Season with salt and pepper. Broil for 5 minutes.
Meanwhile, in a bowl, combine butter and shallot. Spread half of butter mixture on meat. Season with salt and pepper. Without completely removing the baking sheet from the oven, place meat on the other half of the baking sheet. Broil for about 10 minutes or until desired doneness. Stir mushroom mixture a few times during cooking. Place meat on a plate and let rest for about 5 minutes.
Slice meat. Serve and dot with remaining butter. Serve with mushroom mixture and a Mustard Green Salad (see recipe).
Striploin is perfect for this type of cooking. The baking sheet must be very hot to sear meat. This is why we cook vegetables first.