Prep the bread. Slice the loaf in half lengthwise and hollow out the center of both halves to create space for the filling.
Cook the components. Cook the bacon until crisp. Sauté the mushrooms and onions in the same pan. Sear the ribeye steaks, leaving them slightly rare — they'll continue cooking from residual heat once wrapped in the warm bread.
Build the base layer. Place one steak into the hollowed bottom half of the bread.
Sauce it. For a sandwich with two distinct flavor profiles, sauce one half with Worcestershire sauce and the other half with a mix of Dijon mustard and horseradish sauce.
Layer the bacon and cheese. Add a layer of crispy bacon, followed by a layer of Swiss cheese.
Add the mushroom-onion mixture. Pack in as much of the sautéed mushroom and onion mixture as the sandwich will hold.
Add the second steak. Place the second steak on top, pouring in any remaining pan juices for extra flavor.
Finish the layers. Add another light layer of sauce, then top with more Swiss cheese.
Close and wrap. Place the top half of the loaf back on. Wrap the entire sandwich tightly in butcher paper, then in aluminum foil.
Press. Set a heavy cutting board on top of the wrapped sandwich, then add weight on top (the original recipe uses 65kg / 140 lbs). Let it sit for at least 4 hours to compress and allow the flavours to meld.
Serve. Remove the weight, unwrap, slice into portions, and enjoy.