Cut a slice of tomato that is similar in size and width to the mozzarella.
Fresh basil is essential for this dish. If we haven't got huge leaves just use more smaller ones.
A drizzle of olive oil will bind everything together. It's as essential as the basil and as it is with good olive oil, the more expensive the better.
Notes
It's not recommended to make them well ahead as the mozzarella will seep liquid out and ruin our plating. A couple of minutes will be fine but anything over half an hour is risky, especially at room temperature.