Preheat grill to medium.
For the pepper relish:
Grill the Fresno and bell peppers until they are BLACK on the outside. Put in a bowl, cover with plastic wrap and let rest for 10 minutes. Uncover the bowl and allow the peppers to cool. Remove the black char from the outside of the peppers and remove the seeds. Coarsely chop all the peppers and place in a medium bowl. Add the remaining ingredients and combine. Let sit for at least 1 hour before using.*Tip: This is great do ahead thing, or make a big batch and keep it, indefinitely, in the refrigerator.
For the sandwich assembly:
Cut the ciabatta rolls in half lengthwise, leaving 1 long side intact. Scoop out the inside and discard. Combine the vinegar and olive oil in a small bowl and brush on the inside of each roll. Be sure to evenly and thoroughly cover all parts of the bread, (don't miss any corners.) You may have some vinegar-olive oil leftover.
Preheat the oven to 350 degrees F.
Lay 4 pieces of provolone cheese inside each roll. Spread an even layer of the pepper relish on 1 side of each roll. Again, make sure it is an even layer, every bite needs to be the same! Crinkle 2 pieces of ham and gently lay them in a fluffy even layer on the pepper relish. Repeat this process with the mortadella. It is very important that the meat is crinkled and NOT folded in a flat piece. A folded hunk of meat results in a thick dense sandwich that is difficult to eat. Also be sure that the ham and mortadella are evenly distributed throughout the sandwich. Close the sandwich and give a little press to secure. Wrap the sandwich in foil.
To finish the sandwich:
Bake the sandwich in the preheated oven for 15 to 20 minutes or until the cheese is melted and the sandwich is hot all the way through. A panino press is also a great way to heat this up.
Serve on individual plates and enjoy.
A great sandwich is truly a thing of beauty!