1. Remove and discard stems and seeds from ancho chiles; rinse chiles. With scissors, cut chiles into about 1-inch pieces and drop into a blender. Cover with 1 cup boiling water. Let stand until chiles are soft, about 15 minutes, then whirl mixture until smooth.
2. In a 4- to 6-quart pan, combine chile mixture, broth, 1/4 cup water, lentils, and 1/4 teaspoon salt. Cover and bring to a boil over high heat, then reduce heat and simmer, stirring once or twice, until most of the liquid is absorbed and lentils are tender to bite but not falling apart, 20 to 25 minutes. Add more water if lentils get too dry.
3. Meanwhile, set a 10- to 12-inch frying pan over medium-high heat. When hot, add olive oil, bell peppers, onion, and garlic; cook, stirring frequently, until vegetables are tender-crisp to bite, 5 to 7 minutes. Stir in cayenne to taste.
4. Spoon lentils into wide, shallow bowls and top with pepper mixture. Sprinkle with the cilantro and salt to taste. Add a spoonful of sour cream if desired.