Spray a large frying pan with low-calorie cooking spray, and set on a medium heat. Add the onion and garlic, and fry for 5 minutes. Then add the beef mince, and continue to fry for 5 minutes — until browned on all sides.
Add the carrot, courgette, celery, and mushrooms, and cook for a further 5 minutes.
Next, add the chopped tomatoes, tomato purée, water, stock cubes, and mixed herbs. Bring to the boil, then lower the heat and simmer for 20 minutes, until the sauce has thickened and the vegetables are soft. Season to taste, then leave the mixture to one side to cool.
In the meantime, prepare your lasagne sheets. Add the lasagne sheets to a large pan of boiling water and cook for 5 minutes, gently moving them around the pan to stop them sticking together — they should be flexible but not cooked through. Preheat the oven to 180ºC.
Place some of the mince mixture in the bottom of an ovenproof dish (a couple of centimetres deep).
Cut the lasagne sheets in half, lengthways. Spread the halved lasagne sheets with the cooled mince mixture. You want a fairly decent coating, but not too much that you can't roll them up! Starting from the short side, roll up to create a swirl. Then place the rolled up sheets, end up, in the dish. Repeat with the remaining lasagne sheets.
Pour the rest of the mince mixture over the rotolo. Sprinkle with cheese, then bake for 35-40 minutes — until the pasta is cooked and the cheese is golden and bubbling. Enjoy!