Preheat the oven to 350° F.

Heat the oil in a large skillet over medium heat. Add the onions and bell peppers along with 1 teaspoon of salt and sauté for 5-6 minutes, or until the vegetables just begin to soften. Remove from heat and allow the mixture to cool.

Take 12 ounces (about 3/4) of the tortilla chips and break them into smaller pieces.

In a large mixing bowl, combine the cooked onions and peppers, shredded chicken, salsa verde, sour cream, broken-up tortilla chips, and three-quarters of the shredded cheddar cheese. Mix everything together until well combined, ensuring that all the ingredients are evenly distributed.

Transfer the mixture to a 13 x 9-inch baking dish, spreading it out evenly to create a smooth surface. Cover the dish with aluminum foil to prevent the casserole from drying out during baking.

Bake covered for 35 minutes. Remove the foil and add the remaining tortilla chips and shredded cheddar cheese over the top of the casserole. Return the dish to the oven and bake for an additional 5-10 minutes, or until the cheese is melted and bubbly.

Remove from the oven and let cool for a few minutes. Garnish with sliced avocado, if desired, and serve hot, straight from the baking dish.