Grill tortillas until lightly charred on one side; set aside.
In a large skillet (I prefer to use cast iron), heat canola oil over medium high heat.
Add onion and sauté until translucent and lightly browned, about 5 minutes.
Remove onion from skillet; set aside.
Add enchilada sauce to skillet and warm over medium low heat.
Place a thin layer of sauce in the bottom of a 13x9 casserole dish.
Begin assembling enchiladas by dipping a tortilla into the sauce and placing it on a flat surface.
Place 1/8 of the chicken down the center of the tortilla; add 1/8 of the onions and a small amount of cheese on top.
Roll up the enchilada and place it, seam side down, in the casserole dish.
Repeat with remaining tortillas.
Pour remaining sauce over the top the enchiladas in the casserole dish and top with the remaining cheese.
If planning to freeze or make ahead, stop here and cover enchiladas tightly and store in the refrigerator or freezer.
When ready to eat, bake at 350 for 30 minutes.
Top with sliced green onions.