Start by heating olive oil in a large pot over medium heat. Sauté the onion, carrots, and celery until softened and fragrant. Then, add the garlic, harissa paste, and tomato paste, stirring to combine.
Sprinkle in the cumin, cinnamon, and smoked paprika. Stir well for a minute to release their flavors. Then, add the split red lentils and vegetable stock, letting them cook gently until tender.
Remove about a third of the soup and blend until smooth. This step thickens the soup naturally, giving a velvety texture without needing flour or cornstarch.
Return the blended portion to the pot, then add drained chickpeas and baby spinach. Simmer for a few more minutes until the spinach wilts and everything is heated through.
Finish the soup with fresh lemon juice and salt to taste. Ladle into bowls and add a drizzle of extra harissa if desired. Serve hot with warm pitta or flatbread.