Layer 2 pieces of the bread with cheese slices, tomatoes, onion slices and chopped basil. Top both pieces with remaining bread, and brush over the outside of the sandwich with the garlic olive oil.
Cook on a pre heated nonstick pan/skillet over low - medium heat sprayed with cooking oil (or a drizzle of olive oil) until brown and golden on both sides and cheese has melted. You can also cover the pan with a lid to ensure melty cheese.
To serve, drizzle with balsamic vinegar. Enjoy!
Notes
The thicker those mozzarella slices, the more melted cheesy haven you'll have oozing out of your sandwich.