Heat a 12-inch skillet over medium heat and add 2 tablespoons of the olive oil.
Add the diced onion and sauté until softened, about 3–4 minutes.
Season the chicken pieces with kosher salt and freshly ground pepper, then add the chicken to the skillet and increase heat to medium-high.
Brown the chicken on all sides, then add the minced garlic and cook for about 1 minute until fragrant.
Push the chicken to one side of the pan and add the remaining 1 tablespoon olive oil to the empty side.
Add the uncooked rice to the oil and sauté for 2 minutes, stirring so it coats in oil.
Pour in the enchilada sauce, undrained diced tomatoes with green chilies, 1 cup water, and ground cumin; stir to combine.
Bring the mixture to a boil, then reduce heat to low, cover with a tight-fitting lid, and simmer for about 15 minutes, stirring occasionally, until the rice is tender and liquid is absorbed.
If the rice is not done and the pan is dry, add 1/2 to 1 cup more water as needed and continue cooking until tender.
Sprinkle the shredded cheese over the top, cover, and let sit 1–2 minutes until the cheese melts.
Serve topped with desired optional toppings such as sour cream, diced tomatoes, avocado, and cilantro.
