1. Preheat your oven to 200°C/Fan 180°C/Gas 6. Fill your kettle and boil it. Trim the green top off the aubergine, then slice the aubergine in half lengthways. Chop each half into 4-5 wedges and place them in a roomy ovenproof dish. Drizzle 1 tbsp olive oil over the aubergine and dust with salt and pepper. Roast for 30 mins, till tender.
2. Meanwhile, peel the potatoes and chop them into bite-sized pieces. Place them in a large pan and pour in enough hot water from the kettle to cover the potatoes. Add a pinch of salt, pop on a lid and bring to the boil, then turn the heat down and simmer for 10-15 mins till the potatoes are tender when pierced with a fork (slightly too soft is better than slightly undercooked).
3. While the potatoes cook, peel and finely chop the onion. Place a medium-sized pan on a medium heat and add 1 tbsp oil and the onion. Season, then fry, stirring now and then, for 8 mins till glossy and golden.
4. While the onion fries, dice the tomatoes. Peel and grate or crush the garlic. Roughly chop the olives. Pick the thyme leaves off their woody sprigs (push the sprigs through a sieve and the leaves will collect in it). Check the potatoes and drain them if they’re ready.
5. Stir the tomatoes, garlic, olives and thyme into the onion. Cook, stirring, for 10 mins till the tomatoes are soft and pulpy. Taste the sauce and add more salt or pepper if you think they need it.
6. The potatoes should be cooked by now, and ready to be drained. Take the aubergines out of the oven. Set your grill to high. Tip the drained potatoes into the dish with the aubergines and spoon the tomato sauce over the veg.
7. Coarsely grate the mozzarella and scatter it over the veg. Slide the dish under the grill and cook for 5-6 mins till the mozzarella has melted and is golden and bubbling.
8. Meanwhile, whisk 1 tbsp balsamic vinegar with ½ tbsp olive oil and a pinch of salt and pepper. Toss the rocket leaves in the dressing. When the gratin has finished grilling, let it rest for 1-2 mins, then between a couple of warm plates and serve with the rocket salad on the side.