Heat olive oil in a large sauté pan over medium-high heat. Add onions and sauté for about 5 minutes or until translucent. Meanwhile, combine chili powder and tomato paste in a small bowl. Add tomato paste mixture to pan and mix into onions. Cook 2 to 3 minutes, but do not let the chili powder scorch.

Season chicken thighs with salt, place into pan with onions and turn once so they are entirely coated. Add 2 cups chicken broth to pan and lower heat down until mixture reaches a simmer. Cover and cook mixture 30 to 40 minutes (turning chicken once), until chicken is tender when pierced with a knife. Remove chicken from pan and allow to cool. Once cool, shred chicken, discarding any bones, fat and/or gristle. Set shredded meat aside.

Add remaining chicken broth and cilantro stems to pan, stir and simmer uncovered until broth has reduced by one third, about 20 to 30 minutes. Add chicken, diced tomatoes, corn, black beans and lime juice to broth mixture. Stir in fresh cilantro and avocado just before serving.

To serve, ladle into bowls and top with a dollop of sour cream and tortilla chips.