1. Heat oil in a large skillet over medium heat.

2. Add the boneless chicken pieces and cook until browned on both sides.

3. Remove the chicken from the skillet and set aside.

4. In the same skillet, add diced onions and minced garlic. Cook until onions are translucent.

5. Add canned chopped tomatoes, salt, pepper, dried oregano, and red pepper flakes. Stir well.

6. Return the chicken to the skillet and simmer for about 15 minutes, or until the chicken is cooked through.

7. Serve hot with rice or pasta.