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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 chili 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, purée the soup.
Season with sugar, salt, squeeze of lemon.
Serve with plain yogurt, lemon wedges, and warm whole wheat pita bread.