Cut the ciabatta in half and rub the inside of the rolls with olive oil. Cut the garlic clove in half and rub the cut side over the oiled side.

Take a small saucepan and heat the butter in it. Add the onion rings and let them cook slowly over medium to low heat until the onions have become nice and soft. Add the apple cider vinegar and let the onions continue to cook for a little while longer until they are caramelized.

Assemble the sandwich by layering the cheese, arugula, bell pepper, and red onion, finishing with the fresh basil. Toast in the sandwich maker until golden brown and the cheese is nicely melted.