1In a large skillet, add oil and once hot, brown the onion and ground chuck ( chopping into small pieces as it cooks) until beef is no longer pink and onion is tender. Drain and add back to skillet.
2Now, add the tomatoes, chiles, salt, pepper flakes if using, taco seasoning, stir well to combine. Bring to a boil, then simmer for 15 minutes or until most of juice is evaporated, stirring occasionally.
3Preheat oven to 350 degree F.
Spray a 2 quart casserole dish with non stick cooking spray.
Layer half the tortilla chips in bottom of dish, then spread half sauce over chips, next cheese.
Repeat twice - should end with cheese.
4Spread the top with sour cream. Place in oven and bake 30 - 35 minutes.
5Add remaining cheddar cheese and place back into oven until bubbling and golden - 10 - 15 minutes.
6Let stand about 15 minutes before serving.
Can serve with additional sour cream and green onion slices if desired.