Heat your oven to 180°C/Fan 160°C/ Gas 4. Peel and finely chop the onion. Peel and grate the garlic cloves.
Set a frying pan over a medium heat for a few mins. When warm, drizzle in 1 tbsp olive oil and add the onion. Season. Gently cook for 5 mins till softened. Stir in the garlic and cook for 1 min more. Pour in the tin of chopped tomatoes and add 1 tbsp balsamic vinegar (keep the rest for later). Pop a lid on the pan and simmer for 15 mins, stirring now and then.
While the tomato sauce is cooking, slice the courgettes into thin strips lengthways. You should get 6-8 slices per courgette.
Heat a dry frying or griddle pan till smoking hot. Lay some courgette slices in the pan (no need for oil). Cook for 2 mins on each side. They should be slightly charred and softened. Place on a plate. Repeat with the remaining courgettes.
Tear the mozzarella ball into small pieces. Pick the oregano leaves off their stalks.
Taste and season the tomato sauce. Spoon ¼ of the tomato sauce into a small or medium baking dish. Lay some courgette slices on top to cover the sauce. Scatter ¼ of the mozzarella over the courgette. Sprinkle over some oregano leaves.
Keep repeating the layers, finishing with a scattering of mozzarella (keep any leftover oregano for garnishing later). Scatter over the breadcrumbs and finish with a good crack of black pepper. Slide into the oven for 30 mins till the top is golden and the mozzarella has melted.
While the gratin is cooking, whisk the remaining balsamic with ½ tbsp olive oil and some seasoning.
Remove the gratin from the oven. Leave to rest for a couple of mins. Meanwhile, toss the rocket with the balsamic dressing. Scatter the remaining oregano leaves over the gratin and serve it with the rocket salad.