Heat the olive oil in a heavy-bottomed soup pot. Add the onion and cook over medium heat until the onion begins to soften. Add the celery and tomatoes. Stir well and continue to cook for approximately 5 minutes. Add 4 cups water, the lentils, pepper to taste, and half the parsley.

Bring to a boil, reduce heat to a gentle simmer, and cook until the lentils are tender, about 20 minutes. Add water if necessary. Approximately 5 minutes before serving, add the pasta and remaining parsley and salt to taste. Cook at a low simmer, stirring frequently, until the pasta is al dente. Ladle into shallow soup bowls and serve.