For the broth: Heat the oil in a large enameled cast-iron pot over medium-low heat. Add the onion, garlic, leeks, salt, pepper, turmeric, and serrano pepper (if using), and sauté for 35 to 40 minutes until golden. Add the legumes and barley, and sauté for 1 minute.
Pour in the stock and bring to a boil. Reduce heat to low, cover, and simmer for 1½ hours unt