Preheat oven to 350 degrees F.
Bake chicken breasts in preheated oven for 20-30 minutes, depending on the size, making sure the juices run clear when cut into.
Remove from oven and shred with 2 forks. Set aside.
In a large skillet, heat olive oil over medium heat. A
dd onion and let saute for about 5-7 minutes, until it becomes translucent.
Add milk, cream cheese, chiles, enchilada sauce, 1 cup of the cheese, and salt.
Stir well until everything is melted together. Remove from heat.
Spray a 2 quart (or 9x13" baking dish) casserole dish with cooking spray, then place 4 tortillas on the bottom. Spread tortillas with 1/3 of the chicken and 1 1/2 cup of enchilada mixture.
Top with 4 more tortillas, then another layer of chicken and enchilada mixture.
Repeat once more.
Top with remaining 1/2 cup of cheese.
Bake in preheated oven for 35 minutes, then let stand for 10 minutes before cutting.