Preheat the oven to 200°C (gas mark 6).
Heat the olive oil in a large pan and add the onion along with a good pinch of salt.
Cook the onion on a medium heat for 10 minutes until soft.
Add the garlic and cook for a further minute, stirring frequently to ensure the garlic doesn't catch. Add the passata, tomato puree, balsamic vinegar and mixed herbs and simmer for 20 minutes.
Taste and add salt and/or pepper as required.
At the same time, wilt the spinach in a dry pan with the butter.
Drain any excess liquid and roughly chop before placing into a mixing bowl.
Add the ricotta to the bowl with a the freshly-grated nutmeg and season to taste.
Once the tomato sauce has begun to thicken, pour it into a large oven dish.
Lay out each lasgne sheet and spread the spinach and ricotta mix over the whole length of the sheet.
Roll each one up and place into the oven dish with the swirl facing up.
Bake for 30minutes.
Top with the mozzarella and bake for a further 10 minutes until the cheese is bubbling and golden. Garnish with the basil and serve.