Heat oil in a large pan over medium heat. Add onion and cook for 3-4 minutes, until softened and fragrant. Add chicken and the seasonings. Stir for 1-2 minutes. Pour in the tomato sauce, stir, reduce heat to medium-low, and simmer for 3-5 minutes, until sauce has thickened.
Serve warm over tortillas with lettuce, tomatoes, avocado, cheese, or any desired toppings.
Notes
Transfer any chicken leftovers to an airtight container and refrigerate for 3-4 days.