Tortilla soup is the Mexican cousin of chicken soup, except you don't have to be sick to be invited to the party, and your avocado is invited too.
In a blender, puree white onion, garlic, tomatoes, chipotle chile, canola oil, and kosher salt.
In a soup pot, brown chicken thighs both sides, skin side first.
Set chicken aside.
Saute vegetable puree, stirring frequently, scraping the bottom of the pot, until slightly darkened (~10-15 minutes).
Add chicken thighs, chicken stock, white onion, garlic, cilantro, oregano, and kosher salt, bring to a boil, then reduce heat to low, and simmer until the chicken reaches 165 F (~20 minutes).
Discard garlic cloves.
When chicken reaches 165 F, set aside, allowing broth to simmer for an additional 45 minutes. When the chicken is cool enough to handle, shred it, discarding skin and bones.
Add shredded chicken and gently simmer until heated through.
To serve, line the bottom of bowls with tortilla strips, ladle soup on top, and pass garnishes separately.