Heat your oven to 190°C/Fan 170°C/Gas 5. Put a large pan of water on to boil.
Peel and finely chop the onion. Melt the butter over a low heat in a medium pan, then add the onion with a pinch of salt and pepper. Gently cook for 5 mins, stirring now and then, till the onion is soft and glossy.
While the onion cooks, peel and crush or grate the garlic. Pick the leaves off the sage stems then finely chop them. Stir the garlic and half the sage into the onion. Tip in the tinned tomatoes. Add 100ml boiling water. Pop a lid on the pan, turn up the heat and bring to the boil. When the sauce has started to bubble, turn the heat down and simmer for 15 mins to thicken the sauce.
While the sauce simmers, trim the tops off the pattipans then thinly slice the pattipans into ½cm-thick rounds. Halve the rounds to make halfmoons. The water should be boiling by now. Add the pattipan and simmer for 3 mins to just blanch. Drain and leave them to steam dry in the colander.
Drain the mozzarella and coarsely grate it. Arrange half the pattipan slices in the bottom of an ovenproof dish.
Taste the tomato sauce and add a pinch more salt or pepper if you think it needs it. Spoon half the sauce over the pattipan slices. Top with the remaining pattipan, then spoon over the rest of the tomato sauce.
Scatter the grated mozzarella and breadcrumbs over the dish. Slide the gratin into the oven and bake for 15-20 mins, till golden and bubbling.
While the gratin bakes, whisk 1 tbsp balsamic vinegar with 1 tbsp olive oil and season with salt and pepper to make a dressing. Toss the salad leaves with the dressing.
When the gratin is golden and a skewer inserted into it goes through easily (this means the pattipan is tender and cooked through), take it out of the oven and let it rest for 5 mins. Serve the gratin with the salad on the side, garnished with the last of the chopped sage.