In a 4-quart saucepan, bring lentils and water to a boil, and skim froth.
Stir in onion, garlic, ground coriander, cumin, and turmeric, and simmer, partially covered, for 15 minutes.
Stir in carrots and bell pepper, and simmer until carrots are tender, about 10 minutes.
The soup may be prepared up to this point, 3 days ahead, cooled, uncovered, and chilled, covered.
Stir in coriander, scallion greens, cayenne, and salt and pepper, to taste.
Serve soup garnished with coriander sprigs.