Start by melting butter in a skillet over medium heat. Add the diced mushrooms and shallots. Salt well. Cook, stirring often, until the mushrooms have released all moisture and the shallots are well caramelized.
Dice your favorite mushroom for the duxelles.
While the mushrooms cook, split the section of backstrap lengthwise to make two long steaks. Season the meat well with salt, pepper, and garlic powder. Grill for 4-5 minutes per side to desired doneness. Rest the backstrap while you prep the sandwich.
Season the backstrap well before grilling.
Hollow out the bread to fit both sections of backstrap.
Hollow out a loaf of your favorite bread.
Slather horseradish sauce on the inner section of the top and bottom of the bread. Spoon half the mushrooms into the bottom section of the loaf and spread them evenly over the hollowed section.
Spread the loaf with horseradish then add half the duxelles.
Place the grilled backstrap steaks side by side.
Place the grilled backstrap on the sandwich.
Spread the remaining mushrooms over the steaks. Place the top of the loaf onto the sandwich.
Top the backstrap with remaining duxelles and bread.
Wrap the sandwich with wax paper or plastic wrap. Place the wrapped sandwich onto a platter. Place a pyrex dish filled with can goods on top of the sandwich for a press. Refrigerate overnight.
Wrap the sandwich with plastic wrap or paper, then add weight and refrigerate overnight before serving.
To serve, slice the sandwich into cross sections. Serve cold or at room temperature, or warm in an oven to serve hot.