Preheat the oven to 200°C / fan 180°C / gas 6. Mix together the olive oil and za’atar and brush each slice of aubergine on both sides with the oil. Heat a griddle pan over a high heat and sear the aubergine slices for a minute on each side, until they just start to soften and have the distinctive griddle marks. You will need to do this in batches.
Line up eight slices on a lightly greased baking tray and top each slice with a layer of peppers, some of the mozzarella and a scattering of sundried tomatoes. Top each with another aubergine slice to create eight 'sandwiches'.
Bake in the oven for around 10 minutes, until the aubergines are nice and soft and the cheese is oozing. Serve straight away with a green salad.