Heat the olive oil in a large pot and add the onion, celery and carrot with 1/2 teaspoon of salt. Cook gently until the vegetables are very soft and and caramelised, for about 20 minutes.
Mix in the garlic, mushrooms, dried Italian herb mix, fresh rosemary or thyme and nutmeg. Cook for a couple of minutes.
Rinse the lentils very well and then add them to the pot, along with the tomato puree, vegetable stock or water, and 1/2 teaspoon each of salt and pepper. Simmer for 1 hour until the lentils are very tender and the sauce has thickened. Stir the pot occasionally, adding a little more stock or water if necessary.
Add the basil or parsley and taste for salt and pepper. Serve the ragu over your favourite pasta or zoodles, with a sprinkle of parmesan or nutritional yeast flakes.