Preheat the oven to 200°C/gas mark 6. Heat the olive oil in a pan and add the onion. Cook for 10 minutes until soft, then add the garlic and cook for a further minute. Add the passata, tomato puree and balsamic vinegar with some seasoning and simmer for 20 minutes, until thickened.
Meanwhile, heat a large pan over a medium heat. Wash the spinach, then add to the pan and wilt. Place in a sieve over a bowl to get rid of excess water then, once cooled, pat dry with kitchen paper and roughly chop. Add to the ricotta with a pinch of freshly-grated nutmeg and season to taste.
Place a generous teaspoon of the ricotta mixture on a slice of lasagne sheet and spread evenly. Roll this up and repeat with the remaining pasta and ricotta until you have 16 lasagne roll-ups.
Once the tomato sauce has thickened to a good consistency, stir through most of the basil, reserving some to garnish. Pour this into the base of a roughly 21 x 21 cm baking dish and place the lasagne roll-ups in the dish, so they are stood up. Bake for 25-30 minutes, until lightly golden and the pasta is cooked. Garnish with the remaining basil and serve.