Preheat the oven to 350° F. Lightly butter or spray a 9x13 inch baking dish and set aside.
Melt the butter in a small skillet over medium heat. Add in the diced onions and sauté until translucent and beginning to brown, about 5 minutes. Set aside to cool.
Take half of the tortilla chips and crush them into large pieces. Add the pieces to the bottom of your baking dish.
In a large mixing bowl, stir together the cooked chicken, cooked onions, cream of chicken soup, diced tomatoes with green chiles, sour cream, sweet corn, 1 cup of the shredded white cheddar cheese, cumin, chili powder, sea salt, black pepper, and oregano. Add the mixture on top of the chips in the baking dish.
Top with the rest of the tortilla chips followed by the remaining 1 cup of shredded white cheddar cheese.
Cover and bake for 25 minutes. Uncover and bake an additional 10-15 minutes or until the cheese is melted and casserole is bubbly.
Top with diced tomato and green onions before serving.
Notes
*There's a great recipe for easy shredded chicken here.