1-First: preheat the oven to 375°F (190°C). Peel and cube the potatoes evenly for consistent cooking, using those 4 pounds of yellow or russet potatoes cut into 1½-inch pieces.

2-Second: boil the potatoes in salted water until tender, about 15-20 minutes, then drain thoroughly to remove excess moisture.

3-Third: mash the potatoes using a mixer or masher until smooth, then stir in the ½ cup unsalted butter cut into 4 pieces, 2 minced garlic cloves, and 1 cup whole milk for creaminess.

4-Fourth: mix in the 1 cup freshly grated Parmesan cheese, 1½ cups freshly grated mozzarella cheese, and season with salt and black pepper to taste, plus the 2½ tablespoons fresh parsley if you want a fresh touch.

5-Fifth: transfer the mashed mixture to a greased baking dish and spread it evenly for even baking.

6-Sixth: sprinkle an extra layer of grated Parmesan on top to achieve that golden, crispy crust.

7-Seventh: bake uncovered for 20-25 minutes or until the top is golden brown and bubbling. Finally, let it cool slightly before serving, and remember to adapt for vegan options by using substitutes at each step.