Rinse lentils in a sieve under cold running water and drain. In a large saucepan, melt butter over medium heat. Add onions, garlic and curry. Cook, stirring occasionally, until onions have softened, about 5 minutes. Meanwhile, thinly slice carrots and celery. Seed and chop green pepper. Add vegetables to onions as they are cut.
Pour in broth. Stir in lentils, cumin and chili flakes. Turn heat to high and bring to a boil, skimming off any foam that forms.
Reduce heat to low, cover and simmer, stirring often, until lentils soften and soup thickens, about 20 minutes. Covered and refrigerated, soup will keep for several days.
Nutrition (per serving)
Because of the protein content of lentils, this soup makes an energy-boosting meal.