Place the butter/oil in a large pot, add the onion and sauté with a pinch of salt for 5 minutes until golden. Add the celery, garlic and carrots and sauté for another 5 minutes. Stir in the thyme and tomato puree then add in the lentils, stock and can of tomatoes. Cook on a low heat for 30 minutes, adding a little water if it gets dry. Season with salt and pepper.
While this is cooking, cook the linguine as per the instructions on the pack.
Serve together with seasoning and a sprinkle of chopped parsley.