Heat 2 tablespoons of olive oil in a large 12-inch skillet over medium heat. Add the diced onion and cook until softened.
Season the chicken pieces with salt and black pepper.
Increase the heat to medium-high, then add the chicken to the skillet. Cook until lightly browned.
Stir in the minced garlic and cook for 1 minute.
Push the chicken mixture to one side of the pan and add the remaining tablespoon of olive oil to the empty side.
Add the uncooked rice to the oil and sauté for 1 to 2 minutes.
Pour in the enchilada sauce, diced tomatoes with green chilies, water, and cumin. Stir everything together well. This is where the One Pan Mexican Chicken and Rice starts to build its rich flavor.
Bring the mixture to a boil, then reduce the heat to low. Cover with a tight-fitting lid.
Cook for about 15 minutes, stirring occasionally, until the rice is tender and the liquid is absorbed.
Sprinkle the shredded cheese over the top. Cover again for 1 to 2 minutes until the cheese melts.
Finish with sour cream, avocado, cilantro, or diced tomatoes if desired, then serve warm.