Pour the extra virgin olive oil in a medium sized pot and add the finely sliced onion. Cook the onion over low heat for a few minutes, stirring often, until golden.
Add the diced potatoes and a spoon of tomato sauce, which will give a pinkish and inviting colour to the pasta and potatoes. Stir to mix all the flavour, then cover with hot salted water.
Cook over low heat for about 10 minutes, then add the short pasta and the crust of Parmesan cheese and cook for 10 up to 15 minutes, until the pasta is cooked. Stir often once you add the Parmesan crust because the cheese will tend to stick to the bottom of the pan.
When the pasta and the potatoes are cooked through and the cooking liquid has been almost completely absorbed, leaving a thick and creamy soup, remove the pot from the heat and serve with grated Parmigiano Reggiano cheese and a pinch of black pepper or dried chili.