Cook the Mortadella: Heat a large frying pan over medium heat and add olive oil. Place the slices of mortadella in the pan. Fry until crispy on both sides.

Portion and stack the pieces of crispy mortadella on top of each other in the pan and top with provolone cheese.

Add a splash of water (about 1 tsp) to the pan and cover with a lid to create steam. Let it cook until the provolone cheese is melted, about 1 - 2 minutes.

Toast the Buns.

Spread mayonnaise on the bottom half of each toasted bun. Spread Dijon mustard on the top half of each toasted bun. Place the stack of mortadella and melted provolone cheese on the bottom bun. Top with the mustard-spread bun half.

Mix the sparkling water and lemon. Serve over ice if desired.