Plain yogurt, lemon wedges and warm pita, for serving
In a large pot, melt the butter. Add the onion, carrots, celery and garlic and cook over moderate heat, stirring, until softened, 5 minutes. Add the cumin, coriander and chile powder and cook until fragrant, 3 minutes. Add the tomatoes and cook just until softened, 2 minutes. Add the lentils and 8 cups of water and season with salt. Simmer over moderately low heat until the lentils are very soft, 30 minutes.
Working in batches, puree the soup. Season with salt and serve with yogurt, lemon wedges and warm pita.