Preheat your oven to 350. Line a 9 x 13 casserole dish with aluminum foil and spray with cooking spray, set aside.
In a large saucepan, heat the vegetable oil over medium heat. Sauté onions, peppers, and green chilies for 5 minutes or until onions and peppers are softened. Add in garlic and tomato and sauté for another 2 minutes. Stir in the chili powder, cumin, sugar, oregano and salt and pepper and cook until fragrant, about 30 seconds. Stir in the tomato sauce and chicken stock and bring to a simmer. Submerge the chicken breasts into the sauce and poach for about 30 minutes or until the chicken reaches 165 degrees. Remove the chicken from the sauce and set aside to cool. When cool enough to handle, remove the chicken from bone and shred it with a fork.