Put the Caesar salad dressing in a blender or food processor and adding a handful at a time add the romaine to blend together and then blend in the parmesan and lemon zest. It should be the consistency of pesto.
Using a pepper mill, generously coat the top sirloin on all sides with freshly ground pepper. Heat a heavy skillet over high. Add the olive oil to the very hot pan and cook the steak for 2-3 minutes per side for medium rare. Remove from heat and allow the meat to rest at least 5-10 minutes before slicing thinly.
Slice the bread lengthwise almost to the edge and fold the bread open. Spread the Caesar salad mix evenly to the edges, top with the sliced steak and the sliced mozzarella and fold the sandwich together and slice into sections to eat now or tightly wrap it in heavy duty foil or parchment paper and keep chilled until ready to eat. This sandwich holds up well and can be made ahead because the deconstructed Caesar salad won’t wilt.
Any dense, crusty bread will work, such as Ciabatta, Focaccia or any of the Artisanal breads from the bakery section in your local market. I particularly like the Italian Garlic Flat bread that is baked daily at my local market.