Trim the aubergine and dice it into 1cm chunks. Set a heavy-based casserole dish or large pan on a medium-high heat and add 1 tbsp oil and the aubergine. Fry for 5 mins, stirring now and then, till the aubergine is browned all over.
While the aubergine fries, peel and finely chop the onion. Lift the aubergine out of the pan and pop it on a plate. Add the onion with 2 tbsp water and a pinch of salt and pepper. Fry for 3 mins, stirring now and then, till the onion starts to brown.
While the onion fries, peel and grate or crush the garlic. Finely chop the sage leaves.
Stir the aubergine back into the onion with the garlic and most of the sage leaves (keep some back for garnishing). Cook and stir for 1 min.
Stir in the penne. Pour in the cherry tomato passata and stir to mix. Add 1 tbsp red wine vinegar and 1 tsp caster sugar. Drop in the thyme sprigs. Pour in 450ml boiling water, stir a few times to mix it all together and put a lid on the pan (or use a baking tray if you don’t have a lid).
Bring the pasta and sauce to the boil, then turn the heat down to medium-low and simmer for 12 mins till the pasta is almost tender but still a little firm and chalky. Meanwhile, drain and coarsely grate the mozzarella.
When the pasta has cooked for 12 mins, take the lid off the pan and lift out the thyme sprigs. Give the pasta a few stirs. Scatter the mozzarella over the top of the dish. Pop the lid back on and cook for another 3-4 mins till the pasta is tender and the mozzarella has melted.
While the pasta cooks, toss the salad leaves with 2 tsp olive oil and a pinch of salt and pepper. Serve the pasta on warm plates.