Add the lentils, broth, carrots, celery, and onion to your slow cooker.Season with salt. Be generous with the salt, especially if using store bought vegetable broth, which I find to be severely under-seasoned—I use anywhere from 1 to 2 teaspoons. Give everything a good stir.
Set the slow cooker to LOW, and walk away for 8 to 12 hours. Okay, if you happen to be home, of course, you can peek, and give it a stir every few hours. The soup is ready to eat after 8 hours, but those extra 4 hours of simmering add a real depth of flavor (and means it’ll be ready just in time for dinner if you pop the ingredients in the slow cooker at 7:00am).
When ready to serve, ladle the soup into deep bowls. Stir in a bit of pesto, Swirl a bit of olive oil over the soup, and enjoy!