Slice the eggplant into relatively thick slices. Salt the slices lightly and leave it to stand around 5-10 mins then wash off or brush off the salt and pat dry.
If you like, lightly brush the slices with oil or leave as they are and grill a few minutes (or use a grill pan) until you can feel the slices are soft and there are a few grill marks. Remove the slices and allow to cool slightly.
While the eggplant is cooling, mix the oil, crushed garlic and chopped basil. Once the eggplant is cool enough to touch, dice it then toss the eggplant in the oil mixture. Store overnight in the fridge to marinade.
When ready to use, dice the tomatoes in relatively large pieces. Cut the mozzarella into slices then cut the slices in half or quarters. Mix both in with the eggplant and serve (or box it up to take with you - it will keep well a good few hours if kept chilled).