Lentils can be cooked a day ahead. Cool and store them in their liquid. When ready to serve, reheat gently, drain, and serve.
Cover the lentils with water, and let them stand overnight. Drain. In a saucepan, combine ingredients; season with salt; add water to cover by 1 to 2 inches. Bring to a boil. Reduce heat to medium low; cook until the lentils are tender, 10 to 12 minutes. Remove from heat, and discard carrot, celery, and onion; drain and serve.