Preheat and Prepare Bread:
Preheat oven to 400°F. Slice the loaf of focaccia horizontally to open it like a sandwich. Place both halves, cut side up, on a parchment-lined baking sheet.
Layer Pizza Ingredients:
Spread pizza or marinara sauce over both halves. Sprinkle about 3/4 of the Parmesan on top, then layer with sliced mozzarella. Add pepperoni to one side only. Bake for 20 minutes, or until cheese is bubbly and pepperoni is lightly crisped.
Make Caesar Dressing:
Mash garlic and kosher salt into a paste. Transfer to a small bowl, then whisk in anchovy paste, egg yolk, mustard, lemon juice, and Worcestershire. Slowly drizzle in olive oil while whisking to form an emulsion.
Toss Caesar Salad:
In a large bowl, toss romaine with the Caesar dressing, remaining Parmesan, and croutons until evenly coated.
Assemble Sandwich:
Remove focaccia from oven. Pile Caesar salad on the bread half without pepperoni, then close the sandwich. Wrap in parchment, slice into halves or quarters, and serve immediately.