Preheat the oven to 200°C/fan 180°C/gas mark 6. Mix together the olive oil and za’atar and brush the aubergine slices 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 distinctive griddle marks. You will need to do this in batches.
Line up eight slices of aubergine on a lightly greased baking tray and top each one with a layer of peppers, some of the mozzarella and a scattering of sun-dried tomatoes. Top each with another aubergine slice to create eight 'sandwiches'. Bake in the oven for 10 minutes, until the aubergines are soft and the cheese is oozing.