Cook the chorizo in a large stockpot over medium heat, stirring occasionally, for about 5 minutes (or until most of the fat has melted out). Transfer the chorizo to a plate lined with a paper towel. Reserve 1 tablespoon of the remaining fat and discard the rest.
Heat the remaining 1 tablespoon of chorizo fat in the same stockpot over medium-high heat. Add the onion, carrots and celery and sauté for 5 minutes, stirring occasionally. Add the garlic and sauté for 1-2 minutes, until fragrant.
Add 7 cups stock, lentils, potatoes, tomato paste, bay leaf, cooked chorizo, and stir to combine. Continue cooking until the stew reaches a simmer. Reduce heat to medium-low to maintain the simmer. Cover and cook until the lentils and potatoes are tender, about 20 minutes.
Discard the bay leaf. If you would like a brothier stew, feel free to add in a bit of extra stock. Taste the soup and season with salt and black pepper as needed.
Serve warm, garnished with lots of chopped fresh chives.
