Preheat the oven to 400 degrees. On a sheet pan lined with foil, place chicken breasts. Drizzle with olive oil and season well with kosher salt and pepper. Roast for 40 minutes.
In a dutch oven or pot, heat 2 tbs. olive oil over medium heat, then add onions, celery and carrots. Season with salt and pepper and let soften, about 5 minutes. Stir in oregano, garlic and jalapeño and cook another minute before adding tomatoes and chicken stock.
Bring soup to a boil then lower heat to simmer while chicken continues to roast. After chicken has cooked, let rest at least 5 minutes before discarding skin and shredding with forks.
Add chicken and any drippings to the soup and ladle into bowls. Top with avocado, cilantro and any other toppings!