Heat the oil in a heavy soup pot. Add the chorizo, onion, and pumpkin; cook over high heat, stirring, for a few minutes. Add the leeks, garlic, and tomato, and cook for 3 minutes more.
Tie the celery, parsley & bay leaves together w/kitchen string and add along with the lentils to the soup pot. Add enough water to cover the lentils by 2 inches, and bring to a boil. Reduce the heat, and simmer covered for about 20 minutes.
Season with salt and black pepper, add the potato cubes, and simmer until the vegetables are soft, 15 minutes. If necessary, add more water.
Before serving, discard the herb bundle, and taste for seasoning. Serve the soup piping hot, garnished with the parsley.