Unlike most lentils, red lentils cook quickly. This version, a take on the traditional Lebanese soup, is done in the slow cooker. After it's done cooking, it's punched up with a bit of vinegar, puréed, and garnished with onions, cilantro, and a spurt of fresh lemon.
When cooking, even in the slow cooker, be careful. You need to keep an eye on it because when the liquid gets too low it can burn. That's why this isn't a soup you want to jack up on high. It's also best to stay nearby, stirring every hour or so and adding stock as the soup starts to thicken. It's possible you'll need up to four additional cups of stock, so be sure to plan accordingly.
Finished with a bit of acid (red wine vinegar), the soup is puréed until smooth. If you prefer a chunkier texture, skip blending it entirely or use the pulse setting to keep some lentils intact.
When ready to serve, the soup is garnished with red onion, cilantro, and lemon juice.