Heat olive oil in a large pot or Dutch oven (at least 5-quart capacity) over medium-high heat. Add diced red onion and jalapeño and cook until onions soften and turn translucent, about 3 to 4 minutes. They should smell sweet with no raw crunch. Add minced garlic and cook for another 30 seconds until fragrant, being careful not to burn.

Add chicken breasts, fire-roasted corn, and diced green chiles to the pot. Sprinkle in Tajín seasoning, cumin, chili powder, salt, and black pepper. Stir everything together so spices coat the chicken and vegetables evenly.

Pour in chicken stock and bring mixture to a boil over high heat. Once boiling, reduce heat to low, cover the pot, and simmer for 25 minutes until chicken is fully cooked through and reaches 165°F internal temperature. Chicken should be white throughout with no pink remaining.

Remove chicken breasts from pot using tongs and place on a cutting board. Use two forks to shred chicken into bite-sized pieces - it should pull apart easily when fully cooked. Return all shredded chicken to the pot and stir to distribute evenly.

Reduce heat to low and stir in sour cream (or Greek yogurt), shredded Monterey Jack cheese, lime juice, and chopped cilantro. Let soup simmer gently on low heat for 3 minutes, stirring occasionally, until cheese melts completely and everything is well combined into a smooth, creamy consistency. Keep heat low to prevent dairy from curdling.

Ladle soup into bowls and top generously with crumbled queso fresco. Garnish with additional fresh cilantro and serve with lime wedges on the side.