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Tip the onion, celery and carrots into a large non-stick saucepan and add 2-3 tbsp water or stock, if you have some. Cook gently, stirring often, until the vegetables are soft.
Add the garlic, tomato purée and balsamic vinegar, cook on a high heat for 1 min more, add the diced veg, lentils, tomatoes, then bring up to the boil.
Turn to a simmer, then cook for about 20 mins. Meanwhile, cook the pasta following pack instructions, then drain. Season the ragu and serve with pasta and Parmesan on top, if you like.