Slice the red onions thinly and mix with the white vinegar. Set aside for approximately 10-15 minutes until the color intensifies.
Wrap a concrete paver or brick with foil and set aside.
Slice the ciabatta horizontally starting at the short end working your way down the loaf. Hollow out both sides of the ciabatta within ¼-½" all around. [See photo above].* You want to leave a shell but not so thin you can see through the bread or make a hole.
Combine dressing ingredients together in a small bowl.
Using a pastry brush, generously brush the inside of the bread up to the edges. Don't oversaturate the bread.
Beginning with the top side of the bread, start layering the meats and cheeses. I started with the turkey, salami, ham, cheese and then the peppers and onions. I realize this might be backwards but I like the taste at the end of the veggies. It's up to you! [Don't overfill or you won't be able to close the sandwich!]
Put the bottom on the sandwich.
Create a criss-cross of 2 equally sized pieces of parchment paper. Fold up around the sandwich. Repeat with foil.
Place the sandwich in a cast iron skillet or heavy pan. Place a slightly larger pan on top of the sandwich and add your foil covered brick (or gallon of milk or something equally as heavy).
Load this whole contraption into a lower fridge shelf and let it do its thing overnight or longer.
When ready to serve, use a serrated knife to slice the sandwich into equal pieces.