Add oil to large pot or deep skillet and heat to medium heat. Add the onions, garlic,whole serrano, carrots, celery and bell peppers. Season lightly with salt and pepper. Saute for 5-6 minutes.
Transfer tomatoes to the blender. Add a splash of water and blend on high until smooth. Pour tomato puree into pot and cook for 2 minutes.
Add the lentils, broth, cumin and lentils. Bring up to a boil and taste for seasonings. Cook covered at a steady simmer for 10 minutes. Add the potatoes and cilantro and continue cooking for another 30 minutes or until lentils are tender. Yields up to 6 servings.