Heat a skillet over medium heat, and once hot, add the oil to the pan. Add the onions and sauté for about 2 minutes, stirring while they soften. Add the tomatoes, Cajun seasoning, and chicken, then stir and cook for another minute.
Add the white rice and stir to combine with everything in the pan. Pour the water into the skillet, stir, and bring everything to a boil.
Once it's boiling, add the frozen mixed veggies and stir everything. Reduce the heat to the lowest setting you can while you still see it simmer. Cover the skillet and cook until the rice is soft, for about 20 minutes.
Serve:
With a fork, fluff the rice and chicken in the skillet. Serve into bowls.
Notes
Using low-sodium chicken broth will make this dish more flavorful. Or use bouillon paste, about 3 teaspoons, dissolved in water.