Heat your oven to 180°C/Fan 160°C/Gas 4. Slice the tomatoes into wedges and spread them out on a baking tray. Season with salt and pepper and slide into the oven. Roast for 20 mins.
Set a heavy frying pan (preferably ovenproof) over a high heat to warm up. Slice the aubergine in half lengthways and add to the smoking hot pan. Cook for 6-7 mins on each side till very soft. Tip onto a board and cool for a few mins, then scoop the soft flesh out of the blistered skin and transfer the flesh to a food processor.
Drain and rinse the chickpeas. Tip them into the food processor. Peel 1 garlic clove and grate it in, along with the zest from the lemon. Squeeze in the lemon's juice. Add most of the oregano leaves, a little salt and pepper and 1 tbsp olive oil. Blend till smooth. Taste and add a pinch more salt, pepper or a glug more of olive oil if you think it needs it. Set aside.
Wipe the frying pan clean and set it over a medium-high heat. Rub the lamb steaks with ½ tbsp oil and a good pinch of salt and pepper each. Carefully add them to the hot pan and cook for 2-3 mins on each side. Slide the pan into the oven for a further 8-10 mins to finish cooking the steaks. Frying pan not ovenproof? Just transfer the steaks to a baking tray. Once the lamb steaks have finished cooking, pop them on a warm plate, loosely cover with foil and rest for 5 mins.
While the lamb cooks, finely chop the rosemary leaves and tip them into a bowl. Peel and grate in the remaining garlic clove.
When the tomatoes have finished roasting, take them out of the oven, roughly chop them and stir them into the garlic and rosemary, along with a pinch of salt and pepper and 1 tbsp olive oil.
While the lamb is resting, spoon the aubergine and chickpea mash into a small pan and warm over a medium heat for 2-3 mins. Add 1 tbsp of water, if necessary, to loosen the mash up a little.
Spoon the mash onto plates and serve with the lamb leg. Finish with the tomato and rosemary salsa, the remaining oregano leaves and a drizzle of the lamb resting juices.