Stir together the celery, garlic, olives, parsley, olive oil and Italian seasoning in a medium bowl.
Put the bottom half of the ciabatta on a work surface. Arrange half of the cheese on the bread, then top with the salami, turkey, ham and mortadella. Top the meat with the remaining cheese and the olive relish. Cover the sandwich with the top of the ciabatta.
Wrap the sandwich tightly with plastic wrap and place it on a baking sheet. Place another baking sheet on top of the sandwich and weigh it down with a few large cans or a heavy skillet. Refrigerate the sandwich for at least 4 hours and up to 8.
When ready to serve, unwrap the sandwich and cut into 6 to 8 pieces