Heat the olive oil in a large pot over medium heat. Add the onion, garlic and red pepper flakes and cook, stirring occasionally, until the onion is slightly soft, about 2 minutes. Remove 1 tablespoon of the flavored oil and set aside. Add the tomato paste and cook, stirring, 2 more minutes. Add the chicken broth, 4 cups water and the cheese rind, if using. Bring to a boil, then add the pasta and cook, stirring occasionally, 5 minutes. Add the lentils along with the liquid from one of the cans; reduce the heat to medium low and simmer until the pasta is tender, 10 to 12 more minutes. Meanwhile, whisk 1 tablespoon parsley and 1 tablespoon water into the reserved flavored oil. Stir 1/4 cup parmesan and the remaining parsley into the soup; remove the cheese rind (if using) and add salt to taste. Ladle into bowls; drizzle with the flavored oil and top with the remaining 1/4 cup parmesan. Serve with bread, if desired.