Heat the oil in a large skillet over medium heat. Add the onions and peppers and 1 teaspoon of salt and sauté for 5-6 minutes, or until vegetables just begin to soften. Allow mixture to cool.
Break 12 ounces (about 3/4) of the chips into smaller pieces.
In a large bowl, add cooked onions and peppers, shredded chicken, salsa, sour cream, the broken up chips and 3/4 of the cheese and stir to combine.
Add the casserole mixture to a 13 x 9 inch baking dish. Cover baking dish with foil and bake for 35 minutes. Uncover and top with remaining chips and cheese.
Bake uncovered for an additional 5-10 minutes, or until the cheese is melted and bubbly.