Chop onion, carrots, and celery (if using). Start a well-salted medium pot of water to boil. In a second large pot, heat the fat over a medium-high heat. Add the onion, carrots, and celery (if using) and sauté until soft, stirring occasionally. Add the stock and bring to a simmer (note: if you are using a parmesan rind, throw the whole thing in now and then fish it out at the end). Chop any other ingredients that need to be chopped. Add any other vegetables that might need to cook for a little bit (broccoli florets, cabbage, etc.). When the water comes to a boil, add the pasta and cook according to directions. Add the protein (if using) and the flavor enhancers. Continue to simmer. Just before the pasta is done, add any other vegetables that only need a few minutes to cook to the soup (peas, carrots, asparagus, edamame, spinach, kale, etc.). Strain the pasta, return it to its pot, and mix in some olive oil or sesame oil so it won’t stick together. Taste the soup and add whatever extra flavorings it might need. Serve the soup into bowls and then add pasta. Top with grated parmesan, if you like.