Preheat your oven to 400 degrees. Spray a 9 by 13 inch baking dish with cooking spray and set aside.
Heat the olive oil in a small skillet over medium heat. Add the onions and garlic and cook until the onion is translucent about 3 to 5 minutes. Remove from heat.
In a large bowl, combine 3/4 cup of the shredded cheese, the cooked shredded chicken, diced tomatoes, cumin, green chiles, and sautéed onion and garlic.
In the same pan you sautéed the onion in, heat the enchilada sauce. Bring to a simmer, and then reduce the heat.
Pour a 1/4 cup of the warm enchilada sauce in the bottom of the baking dish.
Dip the tortillas in the warm enchilada sauce. Fill with 1/3 cup of the chicken mixture. Roll the tortilla and place seam side down in the baking dish. Continue until all of the tortillas are rolled.
Pour the remaining sauce over the enchiladas. Top with the remaining cheese.
Bake uncovered for 20 minutes or until the sauce begins to boil at the edges.