In a large bowl, mix together the mashed potatoes, ranch dressing, 1 cup of the cheddar, and the ½ cup of parmesan cheese.
Spoon the potato mixture into a 9x13 inch baking dish.
In a medium bowl mix together the crushed crackers and butter.
Sprinkle the cracker mixture, and crumbled bacon evenly over the mashed potatoes.
Cover the dish with foil and bake for 15 minutes.
Remove the foil, place the dish back in the oven and cook for an additional 10 minutes until the topping is golden.
Serve immediately topped with green onions if desired.
NOTE: Once prepared, this casserole can be covered (without topping) and refrigerated for up to 2 days, or frozen for up to 2 weeks. NOTE: If you don't have ranch dressing you can use ranch dressing packet mix, or ⅓ cup of cream cheese or sour cream for more flavor.