Place the peeled and cubed potatoes in a large pot filled with salted water. Bring the water to a boil and cook the potatoes for 15-20 minutes, or until tender.
Step 2: Mashing the Potatoes
After draining the cooked potatoes, transfer them to a large bowl. Add butter, milk, garlic powder, salt, and pepper. Mash until smooth. Stir in the cheddar, mozzarella, and Parmesan cheeses until fully melted and combined.
Step 3: Preparing the Baking Dish
Preheat the oven to 375°F (190°C). Line a 9×13-inch baking dish with parchment paper. Spread the mashed potato mixture evenly and smooth the top with a spatula.
Step 4: Baking
Bake in the preheated oven for 20-25 minutes, or until the edges are golden brown and the top is set.
Step 5: Topping
Let the dish cool slightly, then spread sour cream over the top. Sprinkle with shredded cheddar cheese and chopped green onions.
Step 6: Cutting and Serving
Cut the baked potato mixture into squares. Serve warm and garnish with fresh parsley for a vibrant finish.