Sauté the Vegetables: In a large pot, heat the oil over medium heat. Add the onion, garlic, leek, carrot, and celery. Sauté until the vegetables are tender and fragrant.
Cook the Tomato Base: Stir in the tomato paste and cook for 2 minutes, ensuring it mixes well with the vegetables.
Add the Tomatoes: Add the canned diced tomatoes to the pot. Cook for an additional 5 minutes to deepen the flavors.
Incorporate the Broth: Pour in the chicken broth and stir to combine. Bring the mixture to a boil, then reduce the heat to medium-low.
Add the Main Ingredients: Stir in the cooked beans, shredded chicken, chili powder, oregano, cumin, and black pepper. Let the soup simmer gently for 20-25 minutes, allowing the flavors to meld.
Serve and Garnish: Serve the soup hot, garnished with corn kernels, tortilla chips, sour cream, fresh cilantro, and sliced fresh chiles, if desired.