Heat a large pot over medium-high heat. When hot, add onion and bell pepper; cook, stirring frequently, until vegetables brown and begin to stick to the bottom of the pot, about 6 minutes. Stir in 3 tablespoons of the broth and continue to cook, stirring, until onion is soft and lightly browned. Stir in garlic and chili powder and cook 1 minute, stirring constantly. Add lentils, tomatoes and remaining broth. Bring to a boil, reduce heat to medium-low and simmer, partially covered, 30 minutes or until lentils are almost tender. Uncover and cook 10 minutes longer. Stir in cilantro and serve.