Heat half the oil and the butter in a large frying pan over a medium heat. Add the onions, mushrooms and garlic and cook, stirring, for about 10 mins, until golden and beginning to caramelise. Remove from the pan with a slotted spoon and set aside.
Add the rest of the oil to the pan, and turn up the heat. Season the steaks and cook for 2-3 mins per side, or until done to your liking. Remove from the pan and leave to rest for 4-5 minutes before slicing.
Arrange the leaves over the halved baguette, then top with the onion and mushroom mixture followed by the steak. Spoon on the pepper sauce and sandwich together. Serve immediately.