In a medium bowl, mix together the tomato sauce, vinegar, garlic, chili powder, cumin, oregano, sugar, salt and pepper until thoroughly combined.

In a large skillet or sauté pan, bring the oil to medium-high heat. Add the chopped onions and cook for a couple minutes until they start to soften and become fragrant. Add the chicken breasts in a single layer and cook for 3 minutes on one side, flip them over, and then continue to cook for 3 minutes on the other side. Add the tomato sauce mixture to the pan and bring it to a boil. Lower the heat to medium-low and cover the pan. Simmer for about 20 minutes until the chicken is fully cooked through.

Remove the lid from the pan and transfer each breast to a cutting board, shredding it using two forks and then returning it to the sauce. When all the breasts are shredded, stir them into the sauce until thoroughly combined. Continue to cook for about 5 more minutes until the sauce thickens and soaks into the chicken.