Preheat oven to 325°F; line a sheet pan with foil.
Slice the tomatoes and onion to about 1/4-1/6 of an inch thick.
Wrap the garlic in foil and place on the sheet pan along with the onions and tomatoes.
Drizzle the onions and tomatoes with olive oil and sprinkle with salt and pepper.
Place in the oven and roast for 1 hour.
When done remove from the oven and unwrap the garlic, cut it in half and squeeze all the cloves out of it into a small bowl; stir in the butter.
Take the ciabatta and cut them in halves.
Evenly spread on the garlic butter mixture on both sides of the bread then begin to layer the bottom half with cheese, then pastrami, then beef, then onion, tomato, more cheese and then the top half of the bread.
Wrap the sandwiches in foil and place on a clean sheet pan.
Place another sheet place on top, and weight it down with a big iron skillet.
Place in the 325°F oven and 15-20 minutes.
These sandwiches are great to make ahead of time too!