Start by peeling and cubing 4 to 5 medium russet potatoes. Place them in a large pot of cold water, ensuring they're fully submerged. Bring the water to a rolling boil and cook the potatoes for about 15 minutes or until they're fork-tender.

Once the potatoes are tender, drain them thoroughly and return them to the pot. Add melted butter, sour cream, and milk, then gently mash the mixture until smooth and creamy.

Fold in the shredded sharp cheddar and mozzarella cheeses, alongside the chopped green onions, ensuring everything is evenly combined.

Lightly grease a 9x13-inch casserole dish with cooking spray or butter. Carefully transfer the cheesy potato mixture into the dish, using a spatula to smooth the top evenly.

Preheat your oven to 375°F (190°C). Once heated, place the casserole dish uncovered in the oven and bake for 25 to 30 minutes.

After baking, allow the casserole to rest for about 5 minutes before serving. Garnish with fresh green onions and any optional toppings you'd like.