Heat the butter and olive oil in a large pan and sauté the carrot, onion and celery until they soften. Add the potatoes and stir well.
Add the lentils and the warm stock and cook for about 25 minutes, or until the lentils are tender and the potatoes have broken down. Season with salt and pepper to taste.
While the soup is cooking, prepare the pasta butterflies. Divide the pasta dough into thirds and roll them out to about 2 mm thickness on a lightly floured work surface. Keep the pasta sheets under a damp cloth to prevent them drying out.
Use a crenulated ravioli cutter to cut the pasta into strips about 3 cm x 2 cm. With your thumb and index finger pinch each strip in the middle to create a butterfly (or bow tie) shape.
Cook the pasta butterflies in plenty of boiling salted water. When the lentils and potatoes are soft, add the tomato sauce – this enriches the soup.
Add the cooked pasta butterflies to the soup, which should now have a nice thick consistency. Serve with freshly grated cheese and a good drizzle of extra-virgin olive oil.