Preheat oven to 350 degrees, coat a 9" x 13" casserole dish with non-stick spray then set aside.
Heat oil in a large sauté pan over medium heat until it begins to shimmer. Season chicken lightly with salt and pepper then add to skillet. After cooking for 2 minutes, add the onion and cook until the breasts are cooked all the way through and the onions are translucent. Transfer chicken to a plate then set aside to cool slightly, leaving the onions in the pan.
Add flour to oil in the skillet and whisk to combine. Cook flour in the oil for about 1 minute, then slowly pour in milk, whisking constantly, until no lumps remain. Allow mixture to come to a bubble, then add ½ cup enchilada sauce, a 4 oz can of green chiles, cumin, garlic powder and salt. Momentarily remove the skillet from the heat while you shred the chicken.
When the chicken is ready, return the skillet to medium heat and bring the sauce back to a bubble. Add the chicken, stirring well to combine. When the sauce achieves a bubble, reduce heat to medium-low then simmer about 5 minutes until the mixture becomes very thick. Add 1 cup of cheddar cheese then stir to combine. Remove pan from heat then set aside.
Lightly coat the corn tortillas front and back with cooking spray. Heat each tortilla in a dry nonstick skillet set over medium heat. Cook on both sides just long enough to heat through and completely soften. Once all 12 tortillas have been heated, gather all the components so you can begin assembling the enchiladas.
Divide filling into 12 roughly equal portions and place in the center of the warm tortillas. Roll the tortilla closed then place, seam side down, in the prepared casserole dish. Repeat with remaining filling and tortillas.
Pour the remaining enchilada sauce over the top of the rolled enchiladas and top with remaining shredded cheese.
Bake 15–20 minutes or until hot and bubbly. Remove from the oven and let cool for about 10 minutes before serving.
Even though buying your cheese already shredded is convenient, it’s less expensive to shred your own, time permitting!