Heat grill to high. Cut the ciabatta into bite-size pieces and spread out in a single layer on a baking sheet. Drizzle over 1 tbsp oil. Grill for 3-4 mins, turning the pieces of bread occasionally, until just golden. Leave to cool.
Put the toasted ciabatta in a serving bowl with tomatoes, cucumber, olives and feta. Tear up the basil, then scatter over the salad. Whisk the balsamic and remaining olive oil together and drizzle over, then mix gently to combine.