Heat 2 tablespoons olive oil in a large skillet over medium heat. Add diced onion and sauté until softened.
Season chicken with salt and pepper. Increase heat to medium-high and add chicken to the pan. Cook until lightly browned.
Add minced garlic and cook for 1 minute.
Push chicken to one side, add remaining olive oil, then stir in the uncooked rice. Toast for 1–2 minutes.
Pour in enchilada sauce, diced tomatoes, water, and cumin. Stir well.
Bring to a boil, then reduce heat to low. Cover and simmer for about 15 minutes, stirring occasionally, until rice is tender.
Sprinkle cheese on top, cover, and let melt for 1–2 minutes.
Serve hot with your favorite toppings. Enjoy your One-Pan Mexican Chicken and Rice.