Preheat the oven to 350°F. Grease a 9x13 pan or casserole dish and set aside.*
Place the peeled and cubed potatoes into a large saucepan or pot with a lid; add enough water to cover all the potatoes. Stir in 1 tsp salt.
Place the lid over the potatoes and bring to a boil on the stovetop. Once the water is boiling, turn the burner to low and let the potatoes cook for 15-20 minutes. You can test the doneness of the potatoes by pushing a fork into a potato. If it easily breaks in half, they're good to go!
Pour the boiled potatoes into a strainer and let the water run down the drain.
Dump the potatoes into a stand mixer fitted with the whisk attachment. (If you don't have a stand mixer, you can start with a potato masher and move to a whisk once they're smooth).
Add the butter and whisk until the potatoes are smooth. Add the cream cheese, sour cream, about 2/3 of the shredded cheddar cheese, milk and the green onions or chives.
Whisk until the potatoes are smooth and ingredients are well-combined.
Spread into the bottom of the prepared pan. Top with the remaining cheddar cheese.
Bake for 20-30 minutes or until the cheese is melted and the outer edges begin to brown.*
Remove from the oven and sprinkle with sliced green onions, if desired.