Add in lentils, broth, and water, bring to a boil, then reduce heat to low and simmer, covered, for about 40 minutes.Add the pasta to cook, stirring occasionally, during the last 10 minutes of cooking (the soup will be thick when adding pasta, so you may also add more water if desired to thin the consistency).
Once the lentils and pasta are fully cooked and tender, stir in the spinach and season with salt. Allow to simmer for 2 more minutes to wilt, then remove from heat and serve.