Preheat your oven to 400 degrees.
Peel and chop potatoes, then boil in salted water until soft, about 15-20 minutes. Drain and return to pot or a large bowl.
Ideally, using a handheld mixer, add butter to your potatoes. I add butter first while the potatoes are hot so the butter melts easily.
Add milk, sour cream, ranch (if using), cheese, garlic salt, pepper and any other option you want to include. Blend ingredients together. TASTE your potatoes and add more of whatever you think they need, if anything at all.
Spray a casserole dish with cooking spray and press your potatoes into it. Try and make the top as even as possible.
First, I sprinkle bacon all over the top (if using), then crispy fried onions, and parmesan cheese (if using). Last, I sprinkle between ½ - 1 cup of shredded cheddar and monterey jack cheeses all over the top. Sometimes a little more, sometimes a little less.
Bake for 25-30 minutes until top is a golden brown. Sometimes I will broil for a minute or two to darken the cheese a little more, if needed.
Top with chives or green onions. Serve as desired.