Chicken Enchilada Casserole

1 lb of Shredded Cooked Chicken
1 Small Onion, diced
12 to 14 Small Corn or Flour Tortillas
1 4oz can of Mild Green Chilies
2-1/2 cups of Shredded Mexican Blend Cheese
2 14oz can of Red Enchilada Sauce
1 tsp of Chili Powder
1 tsp of Cumin
1/4 tsp of Dried Oregano
3 Tbsp Vegetable Oil
Salt and Pepper, to taste