1. Brush one side of one piece of bread with olive oil and place on a hot panini press or grill pan. Layer the sliced mozzarella, sliced tomato, sliced red onions and basil leaves, then drizzle with balsamic vinegar and season with just a little salt and pepper.
2. Brush the other slice of bread with the remaining olive oil and place over sandwich, then lower the panini press lid or place a heavy pan on top of the sandwich in the grill. Press down for 20 seconds on medium heat, then leave the pan sitting on the sandwich as it cooks. You will need to flip the sandwich if using the grill press once the bottom is toasted in order to grill the other side. Grill until toasted to a golden brown and the mozzarella starts to melt.