To make ranch avocado sauce: Place avocado, ranch seasoning, sour cream, and ½ cup buttermilk in food processor or blender. Blend (adding more buttermilk if needed). Refrigerate in an airtight container for up to 4 days.
Preheat oven to 350 degrees F. Spray a 9x13 casserole dish with nonstick cooking spray.
In a large bowl, mix together riced cauliflower, chicken, cream cheese, sour cream, ½ cup cheddar cheese, and seasonings.
Transfer to a casserole dish and cover with remaining cheese.
Bake for 30 minutes or until the cheese is golden on the edges and the casserole is bubbling.
Garnish as desired with your choice of cubed avocado, sliced black olives, minced cilantro, diced tomatoes, etc.
Serve with ranch avocado sauce.