Pat lamb dry with paper towels and sprinkle with ¼ teaspoon salt and pepper. Using highest heat, in dutch oven, heat oil until just smoking. Place lamb in pot and cook until well browned about 3 minutes per side; transfer to plate.
Add onion and remaining ½ teaspoon salt to fat left in pot and cook until softened, about 5 minutes. Stir in harissa and flour and cook until fragrant, about 30 seconds. Slowly whisk in broth, scraping up any browned bits and smoothing out any lumps. Stir in lentils, then nestle lamb into pot and add any accumulated juices.
Place in oven and back for 50 mins or until lentils are tender
Transfer lamb to cutting board, let cool slightly, then shred into bite-size pieces; discard excess fat and bones. Stir lamb and chickpeas into soup and let sit until heated through, about 3 minutes. Season with salt and pepper to taste. Top individual portions with tomatoes and sprinkle with cilantro. Serve, passing extra harissa separately.