Heat the olive oil in a large saute pan over medium heat. Add the bacon, if using, and the celery, carrot, onion and parsley. Cook and stir for a minute, then add the potatoes and tomato puree. Cook gently for 30 minutes.
Heat the vegetable stock in a separate pan or in the microwave. Add two ladlefuls of the stock to the vegetable mixture; bring to the boil. Once boiling, add the pasta. Continue adding ladles of hot stock, stirring, cooking until the pasta is tender and the potatoes have broken down, making a creamy sauce.
Sprinkle with grated Parmesan and serve hot.
This recipe was traditionally made with pieces of leftover pasta, so if you don't have any 'Mista Corta' to hand just empty out all of the pasta left in the bottom of your pasta packets and use that. Or break up some spaghetti, which works well too!