Heat the oil in a large, heavy-based frying pan and cook the aubergine slices in batches over a high heat for 1–2 minutes on each side until browned and tender. Remove with a fish slice and keep warm.
Meanwhile, cut the ciabatta loaf in half lengthways, then each half in half again widthways. Place on the grill rack and cook under a preheated high grill, cut-side up, for 1 minute until golden.
Spread each ciabatta toastie with 1 tablespoon of the pesto. Top with the warm aubergine slices, then the tomato slices and finally the mozzarella slices. Drizzle each toastie with 1 tablespoon olive oil, then return to the grill and cook for a further 2 minutes until the mozzarella is melting and beginning to brown in places.