Preheat your oven to 180ºC/350ºF/gas 4. Heat a large saucepan and add a splash of olive oil. Add the leeks and sliced red onions, and sweat, slowly, until soft – about 10 minutes. Add the chopped spinach and briefly cook until wilted down. Set aside to cool slightly, then drain off any excess liquid in the pan.
Mix the ricotta into the leek and onion mixture and season with a tiny pinch of salt and pepper.
Spoon a quarter of the base tomato sauce into the bottom of 6 individual ovenproof dishes, or into a 3-litre ovenproof dish. Cover with sheets of lasagne. Spread half the leek and ricotta mixture over the lasagne, then add a third of the remaining tomato sauce. Repeat with more lasagne sheets, the remaining leek and ricotta mixture and then half the remaining tomato sauce. Finish with a final layer of lasagne sheets, then spread the rest of the tomato sauce over the top.
Tear the mozzarella into small pieces and dot over the top of the lasagne. Sprinkle with the Parmesan. Place on a baking sheet and bake the individual lasagnes for 25–30 minutes or the large one for 45 minutes – they need to be cooked through and golden brown on top.